and Elizabeth Garcia are greatly appreciated for putting up with my strenuous demands as field technicians. ... Dan Gossett, Stephanie Gossett, Elna Black, Tom Zariello, and Aimee ..... Pp. 251-259. In: H.G. Hughes and T.M. Bonnicksen, eds.
NEST-SITE SELECTION, ECTOPARASITES, AND MITIGATION TECHNIQUES: STUDIES OF BURROWING OWLS AND ARTIFICIAL BURROW SYSTEMS IN SOUTHWESTERN IDAHO
by Brian Wade Smith
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Raptor Biology at Boise State University
April, 1999
The thesis presented by Brian Wade Smith entitled Nest-site Selection, Ectoparasites, and Mitigation Techniques: Studies of Burrowing Owls and Artificial Burrow Systems in Southwestern Idaho is hereby approved:
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DEDICATION
This thesis is dedicated to my loving parents, Richard and Brenda Smith, and to my loyally devoted wife, Rebecca.
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ACKNOWLEDGMENTS First, I give thanks to all members of my family for their encouragement and support throughout my academic career at Boise State University. I especially thank my wife Rebecca for her patience with me and her assistance on my project. Second, I thank my friends and fellow graduate students for all of their insights and advice around the office, but especially for all the relaxing and recreational events outside the office. I am particularly grateful for my advisor, Dr. Jim Belthoff, who taught me not only the importance of experimental design and statistical power, but also the art of nymph fishing the Big Wood River. Likewise, I thank my committee members, Dr. Al Dufty and Dr. Steve Novak, for their helpful suggestions, critical reviews, and encouragement during this project. Additionally, Dr. Charles Baker provided sound advice, information about parasites, and fulfilling hunting expeditions. Lara Hannon, Ben Nelson, Hilary Smith, and Elizabeth Garcia are greatly appreciated for putting up with my strenuous demands as field technicians. Extreme gratitude is extended to David Anderson, Trent Brown, Ethan Ellsworth, Sean Finn, Brian Herting, Bob Lehman, Steve Lewis, David Lupien, Rebecca Smith, Dave Oleyar, Liz Vandernoot, and Kurt Zwolfer for helping me dig numerous holes in unforgiving soil for my artificial burrow experiments. I also thank Dr. Mark Fuller and Leslie Carpenter for their technical assistance and cooperation on this project. Identification of ectoparasites was provided by R.J. Adams, Dr. Dale H. Clayton (University of Utah), and Dr. Robert Lewis (Iowa State University, retired). In addition,
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I thank Dr. Marc Bechard, Mike Kochert, Brit Peterson, and Laura Valutis for reporting sightings of burrowing owls. Dan Gossett, Stephanie Gossett, Elna Black, Tom Zariello, and Aimee Pope also provided assistance with this project. Access to owls nesting on private property was granted by the Hayes and Stewart families. Financial and logistical support for this study was provided through challenge cost share grants from the Bureau of Land Management to Dr. Jim Belthoff, by the Department of Biology and Raptor Research Center at Boise State University, and by the Snake River Field Station, Forest and Rangeland Ecosystem Science Center, U.S. Geological Survey, Boise, Idaho. Jim Clark, John Doremus, and John Sullivan facilitated our work in the Boise District and Snake River Birds of Prey National Conservation Area.
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TABLE OF CONTENTS DEDICATION……………...…………………………………………………………….iii ACKNOWLEDGMENTS…...……………………………………………………………iv LIST OF TABLES...…………………………………………………………………….viii LIST OF FIGURES………………………………………………………………………..x GENERAL INTRODUCTION……………………………………………………………1 Study Species: Western Burrowing Owls…………………………………..1 Why Study Nest-site Selection and Ectoparasites? ………………………...3 Overview of Chapters One, Two, and Three……………………………….5 Literature Cited……………………………………………………………..7 CHAPTER 1: EFFECTS OF CHAMBER SIZE AND TUNNEL DIAMETER ON NEST-SITE SELECTION IN BURROWING OWLS ……………………...12 Abstract .………………………………………………………………………….12 Introduction …...………………………………………………………………….13 Overcrowded Hypothesis….………………………………………………16 Study Area and Methods …………………………………………………………18 Study Areas………………………………………………………………..18 Artificial Burrow Placement………………………………………………19 Site Reuse …………………………………………………………………22 Locating and Capturing Burrowing Owls…………………………………22 Measuring and Marking Owls …………………………………………….23 Owl Monitoring …………………………………………………………...23 Data Analyses ……………………………………………………………..24 Results ……………………………………………………………………………25 Artificial Burrow Experiments ……………………………………………25 Effects of Chamber Size …………………………………………..25 Effects of Tunnel Diameter ……………………………………….30 Reuse of Nest Sites ………………………………………………………..35 Discussion ………………………………………………………………………..42 Chamber Choice Experiment……………………………………………...42 Tunnel Choice Experiment ………………………………………………..45 Reuse of Nest Sites ………………………………………………………..46 Conclusions ………………………………………………………………………48
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Literature Cited …………………………………………………………………..49 CHAPTER 2: ECTOPARASITES ON BURROWING OWLS IN SOUTHWESTERN IDAHO: ASSESSMENT OF POTENTIAL EFFECTS ON SITE REUSE AND JUVENILE GROWTH, HEALTH, AND SURVIVAL…………………...54 Abstract …………………………………………………………………………..54 Introduction ………………………………………………………………………55 Study Area and Methods …………………………………………………………58 Study Areas…...…………………………………………………………...58 Locating and Capturing Burrowing Owls…………………………………59 Measuring and Marking Owls …………………………………………….60 Ectoparasite Identification ………………………………………………...60 Levels of Infestation ………………………………………………………61 Juvenile Growth and Body Condition …………………………………….62 Owl Monitoring …………………………………………………………...63 Data Analyses ……………………………………………………………..63 Results ……………………………………………………………………………64 Ectoparasite Identification ………………………………………………...64 Levels of Infestation ………………………………………………………66 Site Reuse …………………………………………………………………66 Juvenile Growth and Body Condition …………………………………….71 Discussion ………………………………………………………………………..73 Ecology of Ectoparasites Collected ……………………………………….73 Levels of Infestation ………………………………………………………78 Site Reuse …………………………………………………………………79 Juvenile Growth and Body Condition …………………………………….80 Conclusions ………………………………………………………………………82 Literature Cited…………………………………………………………………...83 CHAPTER 3: BURROWING OWLS AND HUMAN DEVELOPMENT: RESULTS OF SHORT-DISTANCE NEST BURROW RELOCATIONS TO AVOID CONSTRUCTION IMPACTS……………………………………...89 Abstract …………………………………………………………………………..89 Introduction ………………………………………………………………………90 Study Area and Methods …………………………………………………………92 Study Area ………………………………………………………………...92 Nestling Data ……………………………………………………………...93 Nest Relocation……………………………………………………………94 Results ……………………………………………………………………………97 Nestling Data ……………………………………………………………...97 Nest Relocation……………………………………………………………97 Discussion ………………………………………………………………………..99 Literature Cited …………………………………………………………………102 GENERAL CONCLUSIONS …………………………………………………………..105
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LIST OF TABLES Table 1.1a
Occupancy (1997) of artificial burrow clusters of three placed around 1995 or 1996 natural nest burrows………………...……………………..26
1.1b
Occupancy (1998) of artificial burrow clusters of three placed around natural burrows in which burrowing owls nested between 1995 – 1997……………………….……………………………………………...28
1.2a
Occupancy (1997) of artificial burrow clusters of two placed in suitable burrowing owl habitat and patterns of tunnel-diameter selection for each occupied nest burrow …………………………………………32
1.2b
Occupancy (1998) of artificial burrow clusters of two placed in suitable burrowing owl habitat and patterns of tunnel-diameter selection for each occupied nest burrow …………………………………………33
1.3
Patterns of site reuse and chamber choice within clusters of three artificial burrow systems that owls used for nesting in 1997 and 1998 …………………………………………………………………38
1.4
Patterns of site reuse and tunnel-diameter choice within clusters of two artificial burrow systems that were used for nesting in 1997 and 1998 ……………………………………………………………………40
2.1
Species and demographics of ectoparasites collected from burrowing owls in southwestern Idaho during 1997 and 1998……………………...……….65
2.2a
Patterns of ectoparasite load and site reuse within clusters of two artificial burrow systems that burrowing owls used for nesting in 1997 and 1998 …………………………………………………………………69
2.2b
Patterns of ectoparasite load and site reuse within clusters of three artificial burrow systems that burrowing owls used for nesting in 1997 and 1998 …………………………………………………………………70
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3.1
Nestling information, relocation measurements, and fate of each relocated nest …………………………………………………………….98
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LIST OF FIGURES Figure 1.1
Prediction matrix of research hypothesis ………………………………………...17
1.2
Configurations of artificial burrow systems……………………………………...20
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Mean clutch size and number of fledglings for nests in small, medium, and large artificial burrow chambers…………………………………...31
1.4
Mean clutch size and number of fledglings for nests in 10-cm and 15-cm diameter tunnels ……………………………………………………...36
2.1
Mean ectoparasite levels in 1997 and 1998 by tunnel diameter and chamber size …………………………………………………………………67
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Mean number of ectoparasites per brood counted from owls’ heads and observer’s hand for different tunnel diameters and chamber sizes………………………………………………………………...68
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Mean mass and length of tenth primary per brood as a function of ectoparasite load……………………………………………………………….72
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Mean hematocrit levels of broods with experimentally removed or unaltered ectoparasite levels …………………………………………………..74
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Interaction diagrams for length of tenth primary and mass according to treatment time and level of treatment………………………………………….75
3.1
Location of occupied artificial nest burrows and direction of their relocation within the study site…………………………………………..95
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GENERAL INTRODUCTION This thesis consists of three chapters describing my investigations of nest-site selection, ectoparasites, and nest relocations of burrowing owls (Athene cunicularia) in southwestern Idaho. The purpose of my field research was to (1) determine how chamber size and tunnel diameter influenced selection of artificial burrow systems, (2) identify ectoparasite species of owls and examine their potential effects on nesting success, site reuse, growth, and survival, and (3) investigate the efficacy of short-distance relocations of occupied burrowing owl nests. Information contained in this thesis should be of particular interest to those investigating nest-site selection processes in cavity-nesting species and to those resource managers involved with the active management and conservation of burrowing owls.
Study Species: Western Burrowing Owls Western burrowing owls (Athene cunicularia hypugaea) breed throughout the open and well-drained grasslands, deserts, steppes, prairies, and agricultural lands of western North America, ranging as far north as southern Canada from interior British Columbia east to south-central Manitoba, and as far south as central Mexico (Zarn 1974, Haug et al. 1993). The eastern limit of the breeding range lies roughly along a line from Manitoba to northwest Louisiana (Zarn 1974). Seven other subspecies occur in North and Central America, including the Caribbean Islands (Haug et al. 1993). Most of the
2 birds in North America are migratory or disperse widely, except for populations in southern California, Florida, and the southern plains where owls are partial migrants or sedentary (Coulombe 1971, Haug et al. 1993). Burrowing owls are usually present only during the breeding season (March-October) in Idaho, but their migratory routes and wintering areas remain unknown (King 1996). Burrowing owls typically nest in abandoned mammal burrows where they may lay up to 12 eggs (Haug et al. 1993). In southeastern Idaho, the mean clutch size was 9.9 eggs (N = 30 clutches; Olenick 1990). The mean number of young fledged per nest attempt ranges from 1.6 to 4.9 (Haug et al. 1993). I chose western burrowing owls as study subjects for several reasons. First, they are abundant and usually nest near other pairs (i.e., semi-colonial) in southwestern Idaho, which greatly reduced travel time between nest sites. Next, burrowing owls are experiencing population declines in other regions of North America (Zarn 1974, Collins and Landry 1977, Wedgwood 1978, Martell 1990, Haug et al. 1993, James and Espie 1997). In addition, burrowing owls are tolerant of human and vehicular disturbances at or near their nest sites (Plumpton and Lutz 1993, King 1996, pers. obs.) which was critical given the number of times I needed to visit each occupied nest. Because burrowing owls commonly use artificial burrows (Collins and Landry 1977, Olenick 1990, Botelho 1996), they are an attractive species with which to investigate nest-site selection hypotheses. The use of artificial burrow systems (ABSs) in my study was designed to enhance management techniques for the species while experimentally testing for chamber size and tunnel diameter preferences in burrowing owls. Finally, ectoparasites (e.g., fleas, lice) occur at high densities on some burrowing owl families (J.
3 Belthoff, pers. comm.) and may affect their productivity and behavior. Identifying the role of ectoparasites in burrowing owl survival and reproductive success will enhance management techniques and contribute information about the life-history of this species. In contrast, studying burrowing owls in the field challenged me on a daily basis; their fossorial nature required me to bury the ABSs in the desert soil and my research questions forced me to check occupied burrows on a regular basis. Many logistical problems inherent in studying fossorial organisms were removed with the placement of artificial burrows, but several others seemed to be created (e.g., cattle collapsed several plastic chambers and tunnels).
Why Study Nest-site Selection and Ectoparasites? Nest-site selection can influence reproductive success, microclimate, predation, levels of parasitsm, and many other ecological factors (White and Kinney 1974, Ellis 1982, Birchard et al. 1984, Kern and van Riper 1984, Bekoff et al. 1987, Møller l990, van Riper et al. 1993, Zwartjes and Nordell 1998, Hooge et al. 1999). Therefore, if organisms select an inadequate site, reproductive costs may be expressed as a reduction in survival for parents and/or offspring, future fecundity costs for parents and/or offspring, and costs associated with the timing of reproduction (Nur 1988, Møller 1993, Christe et al. 1996a). Therefore, nest-site selection in cavity nesting species is likely influenced by factors that maximize their fitness and likelihood of survival. The use of ABSs allowed me to address specific nest-site selection hypotheses concerning a secondary cavity nesting species. Previous investigators have used ABSs to compensate for a lack of natural burrows (Collins and Landry 1977), or to study
4 burrowing owl breeding biology (Henny and Blus 1981, Olenick 1990, Botelho and Arrowood 1998), nestling growth rates (Landry 1979, Olenick 1990), and the effectiveness of owl relocation or transplant programs (Turner 1985, Harris and Feeney 1989, Martell 1990, Dyer 1991, Haug et al. 1992, Trulio 1995, Delevoryas 1997, Feeney 1997, Trulio 1997). However, there have been no systematic studies to determine which configurations of ABSs burrowing owls prefer. Thus, the limited success of many these studies may be attributable to unfavorable ABS chamber or tunnel dimensions. Ecologists and evolutionary biologists have paid increasing attention to parasitehost interactions, yet the impact of parasites on their host populations is poorly understood (Toft 1991). Recent studies have found that, in some bird species, ectoparasites reduce clutch size, nestling body mass, and the number of young at hatching and fledging (Møller 1990, 1993, Richner et al. 1993, Møller et al. 1994, Christe et al. 1996a, Dufva and Allander 1996, McFadzen and Marzluff 1996), increase feather preening activities (Clayton 1991), increase mortality (Møller 1990, 1993, Richner et al. 1993, McFadzen and Marzluff 1996), increase nestling begging rates (Christe et al. 1996a) and adult provisioning rates (Tripet and Richner 1997), reduce adult sleeping time (Christe et al. 1996b), and reduce nestling hematocrit levels (McFadzen and Marzluff 1996). In contrast, a few studies found no effect of ectoparasitism on reproductive success or survival in adults or nestlings (Roby et al. 1992, Young 1993, GebhardtHenrich et al. 1998). In southwestern Idaho, ectoparasites occur in various densities in burrowing owl nests (J. Belthoff, pers. comm., pers. obs.). It is unknown what, if any, effects these ectoparasites have on reproductive success, nestling growth rates, and survival in
5 burrowing owls. Also, very few published accounts concerning the diversity of ectoparasites that infest burrowing owls in Idaho exist. With access to numerous broods and adults (through ABS use), I examined the diversity and estimated abundance of ectoparasites on burrowing owls. Additionally, I performed an experiment to investigate potential effects of ectoparasites on nestling growth, survival, and hematocrit levels.
Overview of Chapters One, Two, and Three In Chapter One I present the findings of an artificial burrow experiment that was designed to examine how chamber size and tunnel diameter affect nest-site selection in burrowing owls. I provided two types of artificial burrow clusters: one offered three chamber sizes but a standardized tunnel diameter, and the other offered two tunnel diameters with a standardized chamber size. Upon their return to the study areas, owls presumably selected nest sites based on preferred chamber or tunnel dimensions within the cluster they selected. I monitored reproductive measures (number of eggs, number of young fledged) as a function of chamber or tunnel dimension to examine potential effects of ABS configuration on reproductive success. Burrowing owls preferred the largest chamber size and smallest tunnel diameter available to them in the two different cluster types. Also, significantly more juveniles fledged from the largest chamber than from the second largest chamber in the clusters of three ABSs. This finding is important because most resource managers deploy chambers much smaller than the largest chamber in my experiment. In Chapter One, I discuss hypotheses that may explain the patterns of ABS choice observed in my experiments.
6 Chapter Two focuses on the diversity and abundance of ectoparasites of burrowing owls in southwestern Idaho and their potential effects on site reuse, and nestling growth, survival, and hematocrit levels. I identify the species of ectoparasites I collected from adult and juvenile owls, and describe their relative abundance on a per brood basis. Also, I discuss the results of an experiment I conducted that investigated the effects of ectoparasitism by comparing broods in which I experimentally reduced ectoparasite levels to broods in which natural levels of ectoparasites existed. To reduce ectoparasite levels, I dusted all individuals within a brood and their nests' substrate with an insecticide. I collected three species of flea, one species of lice, and one species of carnid fly from burrowing owls in my study areas. Ectoparasite levels were similar between years of my study and thus, ectoparasite levels had no influence on nest-site reuse between years. Levels of infestation also did not differ between two tunnel diameters or among three chamber sizes used by nesting owls. Ectoparasite level did not affect juvenile owl growth (mass and tenth primary length) or fledging success when analyzing brood averages. In my ectoparasite manipulation experiment, dusted and infested nests did not differ in rates of growth or hematocrit levels. Experimental constraints and the lack of statistical power in my design are discussed in detail. Also, I discuss the ecology and potential effects on burrowing owls of each ectoparasite species I collected. In Chapter Three I present the results of an experiment that examined the efficacy of short-distance relocations of occupied burrowing owl nests. Nests (N = 5) occurred in artificial burrow systems in a field that was being converted from grassland habitat to irrigated agriculture. Nests contained one to five juveniles that ranged from 27 – 45 d.
7 Nest chambers and tunnels were moved (range: 72.5 – 258 m) from original nest areas to selected locations in a buffer strip that surrounded the field. Juveniles were transported to the new sites, but adults were expected to move the short distances on their own. Overall, two families (40%) accepted their relocation sites, two families (40%) exhibited "site tenacity" towards their original nest areas, and one family (20%) dispersed from the immediate vicinity. I describe the characteristics of each successful and unsuccessful relocation. I also introduce hypotheses to potentially explain why certain relocations were or were not successful. I conclude this chapter with a discussion of the efficacy of active and passive relocation efforts, problems I encountered with my experiment, and suggestions to increase effectiveness of mitigation techniques similar to those I employed.
Literature Cited Bekoff, M., A.C. Scott, and D.A. Conner. 1987. Nonrandom nest-site selection in evening grosbeaks. Condor 89:819-829. Birchard, G.F., D.L. Kilgore, and D.F. Boggs. 1984. Respiratory gas concentrations and temperatures within the burrows of three species of burrow-nesting birds. Wilson Bull. 96:451-456. Botelho, E.S. 1996. Behavioral ecology and parental care of breeding western burrowing owls (Athene cunicularia hypugaea) in southern New Mexico, U.S.A. Ph.D. Dissertation, New Mexico State Univ., Las Cruces. Botelho, E.S., and P.C. Arrowood. 1998. The effect of burrow site use on the reproductive success of a partially migratory population of western burrowing owls (Speotyto cunicularia hypugaea). J. Raptor Res. 32:233-240. Christe, P., H. Richner, and A. Oppliger. 1996a. Begging, food provisioning, and nestling competition in great tit broods infested with ectoparasites. Behav. Ecol. 7:127-131.
8 Christe, P., H. Richner, and A. Oppliger. 1996b. Of great tits and fleas: sleep baby sleep. Anim. Behav. 52:1087-1092. Clayton, D.H. 1991. Coevolution of avian grooming and ectoparasite avoidance. Pp. 258-289. In: J.E. Loye and M. Zuk, eds. Bird-parasite interactions: ecology, evolution and behavior. Oxford Univ. Press, New York. Collins, C.T., and R.E. Landry. 1977. Artificial nest burrows for burrowing owls. N. Amer. Bird Bander 2:151-154. Coulombe, H.N. 1971. Behavior and population ecology of the burrowing owl, Athene cunicularia, in the Imperial Valley of California. Condor 73:162-176. Delevoryas, P. 1997. Relocation of burrowing owls during courtship period. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:138-144. Dufva, R., and K. Allander. 1996. Variable effects of the hen flea Ceratophyllus gallinae on the breeding success of the great tit Parus major in relation to weather conditions. Ibis 138:772-777. Dyer, O. 1991. Reintroduction of burrowing owls (Athene cunicularia) to the South Okanagan Valley, British Columbia (1983-1988). Provincial Mus. of Alberta Nat. Hist. Occasional Paper No. 15. Ellis, J.H. 1982. The thermal nest environment and parental behavior of a burrowing bird, the bank swallow. Condor 84:441-443. Feeney, L.R. 1997. Burrowing owl site tenacity associated with relocation efforts. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:132-137. Gebhardt-Henrich, S.G., P. Heeb, H. Richner, and F. Tripet. 1998. Does loss of mass during breeding correlate with reproductive success? A study on blue tits Parus caeruleus. Ibis 140:210-213. Harris, R.D., and L. Feeney. 1990. Restoration of habitat for burrowing owls (Athene cunicularia). Pp. 251-259. In: H.G. Hughes and T.M. Bonnicksen, eds. Restoration ’89: the new management challenge. Proceedings of the society for ecological restoration first annual meeting. The Univ. of Wisconsin Arboretum, Madison.
9 Haug, E.A., D. Hjertaas, S. Brechtel, K. De Smet, O. Dyer, G. Holroyd, P. James, and J. Schmutz. 1992. National recovery plan for the burrowing owl. A report prepared for the Committee for the Recovery of Nationally Endangered Wildlife. Canadian Wildlife Federation, Ottawa. Haug, E.A., B.A. Millsap, and M.S. Martell. 1993. Burrowing owl (Speotyto cunicularia). In: A. Poole and F. Gill, eds. The Birds of North America, No. 61. The Academy of Natural Sciences, Philadelphia; The American Ornithologists' Union, Washington, D.C. Henny, C.J., and L.J. Blus. 1981. Artificial burrows provide new insight into burrowing owl nesting biology. Raptor Res. 15:82-85. Hooge, P.N., M.T. Stanback, and W.D. Koenig. 1999. Nest-site selection in the acorn woodpecker. Auk 116:45-54. James, P.C., and R.H.M. Espie. 1997. Current status of the burrowing owl in North America: an agency survey. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:3-5. Kern, M., and C. van Riper III. 1984. Altitudinal variations in nests of the Hawaiian honeycreeper Hemignathus virens. Condor 86:443-453. King, R.A. 1996. Post-fledging dispersal and behavioral ecology of burrowing owls in southwestern Idaho. Unpubl. M.S. Thesis, Boise State Univ., Idaho. Landry, R.E. 1979. Growth and development of the burrowing owl. Unpubl. M.S. Thesis, California State Univ., Long Beach. Martell, M.S. 1990. Reintroduction of burrowing owls into Minnesota: a feasibility study. Unpubl. M.S. Thesis, Univ. of Minnesota, Minneapolis. McFadzen, M.E., and J.M. Marzluff. 1996. Mortality of prairie falcons during the fledging-dependence period. Condor 98:791-800. Møller, A.P. 1990. Effects of parasitism by the haematophagous mite on reproduction in the barn swallow. Ecology 71:2345-2357. Møller, A.P. 1993. Ectoparasites increase the cost of reproduction in their hosts. J. Anim. Ecol. 62:309-322. Møller, A.P., F. de Lope, J. Moreno, G. Gonzalez, and J.J. Perez. 1994. Ectoparasites and host energetics: house martin bugs and house martin nestlings. Oecologia 98:263-268.
10 Nur, N. 1988. The cost of reproduction in birds: an examination of the evidence. Ardea 76:155-168. Olenick, B.E. 1990. Breeding biology of burrowing owls using artificial nest burrows in southeastern Idaho. Unpubl. M.S. Thesis. Idaho State Univ., Pocatello. Plumpton, D.L., and R.S. Lutz. 1993. Nesting habitat use by burrowing owls in Colorado. J. Raptor Res. 27:175-179. Richner, H., A. Oppliger, and P. Christe. 1993. Effect of an ectoparasite on reproduction in great tits. J. Anim. Ecol. 62:703-710. Roby, D.D., K.L. Brink, and K. Wittmann. 1992. Effects of bird blowfly parasitism on eastern bluebird and tree swallow nestlings. Wilson Bull. 104:630-643. Toft, C.A. 1991. Current theory of host-parasite interactions. Pp. 3-15. In: J.E. Loye and M. Zuk, eds. Bird-parasite interactions: ecology, evolution and behavior. Oxford Univ. Press, New York. Tripet, F., and H. Richner. 1997. Host responses to ectoparasites: food compensation by parent blue tits. Oikos 78:557-561. Trulio, L.A. 1995. Passive relocation: a method to preserve burrowing owls on disturbed sites. J. Field Ornithol. 66:99-106. Trulio, L.A. 1997. Strategies for protecting western burrowing owls (Speotyto cunicularia hypugaea) from human activities. Pp. 461-465. In: J.R. Duncan, D.H. Johnson and T.H. Nicholls, eds. Biology and conservation of owls of the northern hemisphere: proceedings of the second international owl symposium. U.S. Dept. of Agric. Gen. Tech. Rept. NC-190. Turner, J. 1985. The burrowing owl transplant program in the South Okanagan Valley of British Columbia. Unpubl. Rept. for JT Biotech Associates, Inc., Princeton, British Columbia. 63 pp. van Riper III, C., M.D. Kern, and M.K. Sogge. 1993. Changing nest placement of Hawaiian common amakihi during the breeding cycle. Wilson Bull. 105:436-447. Wedgwood, J.A. 1978. The status of the burrowing owl in Canada. A report prepared for the Committee on the Status of Endangered Wildlife in Canada. Canadian Wildlife Service, Ottawa. White, F.N., and J.L. Kinney. 1974. Avian incubation. Science 186:107-115.
11 Young, B.E. 1993. Effects of the parasitic botfly Philornis carinatus on nestling house wrens, Troglodytes aedon, in Costa Rica. Oecologia 93:256-262. Zarn, M. 1974. Burrowing owl, Athene cunicularia hypugaea. Report No. 11, Habitat management series for unique or endangered species. Bureau of Land Mgmt., Denver, Colorado. 25 pp. Zwartjes, P.W., and S.E. Nordell. 1998. Patterns of cavity-entrance orientation by gilded flickers (Colaptes chrysoides) in cardón cactus. Auk 115:119-126.
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CHAPTER 1: EFFECTS OF CHAMBER SIZE AND TUNNEL DIAMETER ON NEST-SITE SELECTION IN BURROWING OWLS
Abstract Using field experiments, I examined choice for artificial burrow configuration by nesting burrowing owls (Athene cunicularia) in southwestern Idaho during 1997 – 1998. To assess potential preference for chamber size, I placed clusters of three artificial burrows around natural nest sites used in 1995, 1996, and 1997. Each cluster contained three different chamber sizes (small, medium, and large), each with a standard tunnel diameter. To assess potential choice for tunnel diameter, I placed clusters of two artificial burrows in suitable burrowing owl habitat. Each of these clusters offered two tunnel diameters (10 and 15 cm), each with a standardized chamber. Overall, 77 clusters of three artificial burrows were available for use by nesting pairs of owls during the two years of this study. In 1997, nesting pairs of burrowing owls occupied 18 clusters; 16 pairs used large, one used medium, and one used small chambers. In 1998, twenty-seven clusters were occupied by nesting pairs, with 16 pairs using large, six using medium, and five using small chambers. The distribution of use in each year, as well as for both years combined, differs significantly from uniform and indicates that burrowing owls prefer to nest in artificial burrow systems with large nest chambers. Despite such preference,
13 there was no effect of chamber size on clutch size, but the number of fledglings per nest was significantly greater in large chambers when compared with medium chambers. In 1997 and 1998, seventy-two clusters of two were available. Overall, owls occupied 44 of these; 30 pairs used burrows with 10-cm diameters and 14 pairs used burrows with 15-cm diameter tunnels. This distribution of use also differs significantly from uniform and indicates that burrowing owls prefer to nest in burrows with small-diameter tunnels. Thus, my results indicate strongly that burrowing owls prefer to nest in artificial burrows with (1) large chambers and (2) small-diameter tunnels. Burrowing owls may select artificial burrows with these configurations to reduce negative effects of overcrowding, to gain the most stable microclimate for developing juveniles, or to deter large grounddwelling mammalian predators.
Introduction Relatively few bird species nest underground, possibly because of the increased risk of nest failure. Underground nests are susceptible to flooding or to ground-dwelling predators (Coulombe 1971, Haug et al. 1993). On the other hand, the burrow environment is a climatically stable nesting environment that protects birds from temperature extremes (White et al. 1978, Ellis 1982). Despite such benefits, the availability of underground nest sites may be limited depending on whether a species digs its own burrow (bank swallow, Riparia riparia, Ellis 1982; Florida burrowing owl, Athene cunicularia floridana, Haug et al. 1993, Millsap and Bear 1997) or relies on other fossorial organisms or geological cavities for nest burrows (western burrowing owl, Athene cunicularia hypugaea, Haug et al. 1993). For those species that rely on pre-
14 existing burrows, nest-site selection depends on burrow availability in suitable habitat (Dyer 1991, Botelho and Arrowood 1998), and selection influences the success and productivity of the nesting attempt (Coulombe 1971). Western burrowing owls have declined significantly throughout much of their range as a result of agricultural conversion, destruction of colonial rodents and other burrowing mammals, and urbanization (Zarn 1974, Dyer 1991, Haug and Didiuk 1993, Trulio 1995, James and Espie 1997). For this reason, they are listed as federally endangered in Canada and have special status (endangered, threatened, or species of special concern) in 11 central or western U.S. states (James and Espie 1997). To compensate for the lack of natural burrows or to reintroduce burrowing owls to former portions of their range, wildlife managers have deployed artificial burrow systems (ABSs). Collins and Landry (1977) developed the first documented ABS study in California. They used ABSs to eliminate the possibility of nest burrow collapse and to entice owls to nest in areas not previously used (because of a lack of natural burrows) and that were protected from disturbance. Since then, ABSs have been used to study burrowing owl breeding biology (Henny and Blus 1981, Olenick 1990, Botelho and Arrowood 1998), nestling growth rates (Landry 1979, Olenick 1990), and the effectiveness of owl relocation or transplant programs (Turner 1985, Harris and Feeney 1990, Martell 1990, Dyer 1991, Haug et al. 1992, Trulio 1995, Delevoryas 1997, Feeney 1997, Trulio 1997; also see Chapter Three). Burrows typically offer a micro-environment that is significantly different from ambient conditions (Ellis 1982, Birchard et al. 1984) in terms of temperature and gas concentrations (e.g., CO2 and O2). Because western burrowing owls rely on burrows dug
15 by fossorial mammals, the dimensions of the tunnel and nest chamber vary depending on the original excavator, which can affect the microclimate of the underground system (Ellis 1982, Birchard et al. 1984). Western burrowing owls, which are considered secondary cavity nesters because they usually do not dig their own burrows, commonly modify tunnels and probably chambers of natural burrows (Thomsen 1971, Zarn 1974). These modifications may enhance characteristics within burrow systems, but it remains unknown what attributes burrowing owls seek within their burrows to meet their preferences for a nest burrow. Understanding their requirements for nest and roost burrows may be an essential component in burrowing owl management by providing insight into ecological or physiological constraints placed on underground nesting species. While information on the above-ground features of burrowing owl nest sites is available in the literature (Rich 1986, Plumpton and Lutz 1993), there is virtually no information on below-ground features of burrows important to nesting owls (Haug et al. 1993). Furthermore, no previous ABS studies have compared the effects of chamber or tunnel configurations on reproductive success in burrowing owls and their use of ABSs. Collins and Landry (1977) speculate that actual ABS chamber dimensions are not critical as long as one right-angle turn occurs in the tunnel. Their design, a 30-cm x 30-cm x 20cm wooden chamber with a 10-cm tunnel, has been used extensively (Landry 1979, Henny and Blus 1981, Harris and Feeney 1990, Martell 1990, Trulio 1995). Some investigators have used 19-L (5 gal) plastic buckets for the nest chamber (Dyer 1991, Botelho 1996, D.A. Beig unpubl. data). Only Dyer (1991) has used both wooden and plastic bucket ABSs in the same study, but only because the wooden chambers
16 deteriorated quickly. Therefore, despite the claim by Collins and Landry (1977), it remains unknown if chamber dimension affects the selection of ABSs or the reproductive success of western burrowing owls using ABSs. The objective of my study was to examine the effect of three chamber types (small, medium, and large) and two tunnel diameters (small and large) on nest-site selection by burrowing owls in a field experiment. Also, I wanted to compare the reproductive success (i.e., clutch size and number of young fledged per nesting attempt) among owls nesting in the different ABS types. Given that burrowing owls produce such large clutches of eggs (range up to 12, Haug et al. 1993), it seems logical that available space in both chambers and tunnels might affect nest-site choice. Thus, I designed my experiments to test what I termed the Overcrowded Hypothesis for nest-site selection. Overcrowded Hypothesis Because burrowing owls commonly have clutch sizes reaching 12 eggs, they may prefer large chamber sizes and tunnel diameters to accommodate many nestlings (Fig. 1.1). During the nestling stage, young owls commonly emerge from the burrow in the mornings and evenings but avoid mid-day heat by staying within the burrow system (Coulombe 1971). Moreover, nestlings often return to the burrow once they acquire several prey items, making the entire burrow system an important location for digestion, rest, and protection from predators (Botelho 1996). Although fledging can occur at 30-45 d post-hatching (Haug et al. 1993), burrowing owl broods remain near and continue to inhabit natal and nearby satellite burrows until they disperse (Zarn 1974, King 1996). This hypothesis predicts that the long period of dependence on the natal burrow biases an adult pair’s selection process towards a burrow system with a large chamber size and
17 A. EFFECTS OF ABS CHAMBER SIZE
Overcrowded Hypothesis Small and Medium Chambers
Rarely selected for nesting
Large Chamber
Often selected for nesting
B. EFFECTS OF ABS TUNNEL SIZE
Overcrowded Hypothesis Small-diameter tunnel
Rarely selected for nesting
Large-diameter tunnel
Often selected for nesting
Figure 1.1. Prediction matrix of research hypothesis. Predictions of the Overcrowded Hypothesis for effects of (A) chamber size and (B) tunnel diameter on nest-site selection in artificial burrow systems.
18 tunnel diameter to accommodate activities of even the largest broods. Therefore, the Overcrowded Hypothesis predicts that large ABS chambers and large-diameter tunnels will be selected more often than the two smaller ABS chambers or the smaller-diameter tunnel.
Study Area and Methods Study Areas I examined the Overcrowded Hypothesis and nest-site selection by burrowing owls in two study areas in southwestern Idaho. The first was located in Ada County, approximately 3.2 km south of Kuna (43° 25' N, 116° 25' W) and 23 km north of the Snake River Canyon. Vegetation in this area is characterized by big sagebrush (Artemisia tridentata) shrubland and disturbed grasslands dominated by cheatgrass (Bromus tectorum) and tumble mustard (Sisymbrium altissimum). Surrounding areas contain irrigated agricultural fields (primarily alfalfa, mint, and sugar beets), scattered residential homes, and several large dairy farms. The topography is flat to rolling with elevations ranging from 841 m to 896 m. Rock outcrops and a few isolated buttes (e.g., Kuna Butte, elev. 986 m) exist in the region. Temperatures range from -20° to 45°C, and annual precipitation averages less than 20 cm (NOAA 1985). In this area there is a relatively high density of burrows excavated by American badgers (Taxidea taxus). Burrowing owls commonly use badger burrows for nesting and shelter throughout the breeding season, especially if the burrows are near agricultural fields (Gleason 1978, Leptich 1994, pers. obs.).
19 My second study area was in Elmore County, approximately 8 km north-northeast of Grand View (43° 00' N, 116° 00' W) and adjacent to State Highway 67. This area is a mosaic of irrigated agriculture and disturbed grasslands. Elevations range from 853 m to 922 m. The area contains very few homes, several paved and dirt roads, and an electrical substation. The Snake River is approximately 7 km south-southwest of this study area. Temperatures here range from -29° and 43°C, and precipitation averages 26 cm per year (NOAA 1985). Both study areas were once typical shrubsteppe communities dominated by large expanses of big sagebrush (Hironaka et al. 1983). Range fires and other disturbances have converted much of the shrublands to exotic annual communities dominated by cheatgrass and tumble mustard. In general, the Kuna Butte study area contains more native shrubland than the seemingly more disturbed Grand View study area (Belthoff and King 1997). Artificial Burrow Placement Before burrowing owls arrived from wintering areas, I deployed clusters of two and three artificial burrow systems in or around the two study areas. The clusters of three artificial burrows tested for chamber size preferences and encircled natural burrows that owls used for nesting in 1995, 1996, or 1997 (Belthoff and King 1997, Belthoff and Smith 1998; Fig. 1.2). Within clusters of three, each artificial burrow consisted of a 15cm diameter tunnel made of flexible, perforated plastic pipe and a plastic nest chamber. Each cluster had chambers of three sizes: a 30-cm x 30-cm x 20-cm (17-L; 4.5-gal) plastic container, a 19-L (5-gal) bucket with a 30-cm diameter, and a 50-cm x 35-cm x 40-cm (68-L; 18-gal) plastic tub. These chamber dimensions fall within the range of
20 A.
Small
5m
Medium
Natural Burrow
5m
Large
5m
B.
Small-diameter tunnel
Large-diameter tunnel
3m
Figure 1.2. (A) Configuration of chambers around natural burrows for the chamber choice experiment and (B) configuration of chambers for the tunnel diameter choice experiment (see text for explanations of both).
21 dimensions of natural burrows that owls have used for nesting (Haug et al. 1993). All entrances within a cluster were equidistant (5 m) from and were oriented in the same direction as the historical nest burrow entrance, and tunnel entrances were 120 degrees apart from each other. I randomly assigned chamber size to locations in each cluster. All ABS tunnels were 2 m long with a 90-degree turn between the entrance and the ABS chamber. Each tunnel sloped downward (20-30 degrees) toward the chamber within the range typical of nest burrows within both study areas (Belthoff and King 1997). The tunnel inserted into the chamber on a level plane, and the top of each ABS chamber was at least 30 cm underground. To increase the probability of ABS use during my study, I (1) supplied each chamber with approximately 3 cm of fresh soil (obtained while digging holes for ABS placement) for nesting substrate, (2) placed a wooden perch in the center of the cluster (as in King 1996), (3) created dirt mounds at ABS entrances to mimic those of natural burrows, (4) scattered debris (i.e., manure, prey remains, pellets) from previous nests onto the entrance-mounds, and (5) used rocks to block entrances to the historical nest burrow and any other suitable burrow within a 10-m radius of the historical nest burrow to prevent their use. The rocks were removed after juveniles fledged so the previously blocked burrows could be used as refuge burrows (satellites) if desired. I placed clusters of two artificial burrows in suitable habitat that lacked nesting pairs of burrowing owls (rather than around historical nests as in the chamber choice experiment). Clusters of two were designed to test for preference of tunnel diameter (Fig. 1.2). One artificial burrow had a 15-cm diameter tunnel, the other had a 10-cm diameter tunnel, and chamber size was standardized (19-L plastic buckets for both chambers). I placed the two burrows adjacent to one another with 3 m between each entrance. I
22 oriented tunnel entrances in a south-southeast direction and a north-norteast direction in the Kuna Butte and Grand View study areas, respectively, which is typical for orientation of natural nest sites in these areas (Belthoff and King 1997). I placed a wooden perch between the tunnel entrances in all clusters of two and used the same tunnel lengths and slopes, chamber depths, and general methods as those for clusters of three. Site Reuse If owls used a cluster of three for nesting in 1997, I rearranged the position of chambers within the cluster before owls returned in 1998. In doing so, I stipulated that the chamber used for nesting in 1997 could not be placed in the same location as the previous year. Doing this allowed me to make certain that owls chose ABSs based on chamber size rather than a specific site within a cluster. In addition to the rearrangement, I cleaned all chambers and tunnels of any material that remained from 1997 and supplied them with fresh soil as nesting substrate. If owls used a cluster of two for nesting in 1997, I switched the location of the ABSs within the cluster. This also allowed me to determine if owls chose an ABS based on tunnel diameter or a specific site within a cluster. Again, I supplied each tunnel and chamber with fresh soil after removing all materials that remained from 1997. Locating and Capturing Burrowing Owls To locate nesting owls, I revisted historical nest sites (surrounded by ABSs in 1997) and searched suitable habitat in both study areas on foot and from automobiles. I performed most surveys during daylight hours. After locating owls, I regularly monitored their nesting activities. In 1998 I placed new clusters of three ABSs around nests found at natural burrows in 1997.
23 To capture owls I used Havahart® traps and noose rods as described in Belthoff et al. (1995) and King (1996). I also used one-way basket traps to capture adults as they departed artificial burrows. These traps consisted of a 0.5-m section of flexible plastic pipe (10-cm diameter), a small piece of transparent Plexiglas®, and an enclosure made of “chicken wire” with 2.5-cm diameter openings. The Plexiglas® was fastened to one end of the pipe by a hinge from above which allowed the door to swing upwards when an owl attempted to exit the pipe. This end of the pipe was inserted into the wire basket. The open end of the pipe was inserted into artificial burrow tunnels when I needed to determine the status of a nest in an ABS. Digging down to the artificial chamber caused any adults in the nest chamber to enter the basket and become trapped when the hinged door closed behind it. Measuring and Marking Owls Upon capture, I recorded each owl's mass (to the nearest gram), wing length, tarsus length, tail length, and length of exposed culmen (all to the nearest 0.5 mm). I classified adult owls as female if they had a well-developed brood patch. I classified the remainder of adults as males based on their lack of a brood patch, relatively lighter plumage, and behavior near nests (Thomsen 1971, Zarn 1974). I could not discern sex of young owls based on appearance or morphological measurements. I fitted all owls with a United States Geological Survey aluminum leg band (size 4) and three plastic, colored leg bands (National Band and Tag Co., Newport, KY) for future identification. Owl Monitoring I conducted follow-up visits at nests in artificial burrows to determine if they were successful. For owls nesting in ABSs, I determined minimum number of eggs produced,
24 nestling survival, and the number of young fledged. Successful nests in ABSs had at least one young owl survive to fledging age (> 28 d). Data Analyses I performed chi-squared goodness of fit tests to examine choice of nest chamber size or tunnel diameter by nesting owls. I used a similar test to examine site reoccupancy within an ABS cluster. Using analysis of variance (ANOVA), I also examined effects of chamber size and tunnel diameter on number of eggs and fledglings. When the ANOVA indicated significant differences existed, I performed pair-wise mean comparisons using the least significant difference (LSD) procedure. Because some clusters in both experiments suffered damage which rendered them unavailable to owls choosing sites within a cluster, I eliminated them from analyses of choice (chi-squared analyses) but included them in analyses of productivity (i.e., mean number of eggs and fledglings) by chamber or tunnel dimension. Additionally, I included one cluster of three in the analysis of choice but eliminated it from analyses of productivity because I accidentally disturbed the nest when the eggs were hatching, and only three of nine apparently viable eggs hatched. In 1998, six owl pairs attempted a second nest after their first attempt failed. I used first nesting attempts in all analyses and used the second attempts (1) in choice analyses only if the pair re-nested at a new cluster and (2) in all analyses of productivity. Because two dependent variables (eggs, young) were collected from the same experimental units and analyzed, I used Bonferroni adjusted P-values (Zar 1996). Results were considered significant when P < 0.025 for productivity analyses. Otherwise, results were considered significant when P < 0.05. All means are given as x ± SE.
25 RESULTS Artificial Burrow Experiments Effects of Chamber Size I assessed preference for nest chamber size using clusters of three artificial burrows placed around historical nesting sites. In 1997, I placed 34 clusters of three (N = 17 in each study area), of which nesting owls used 21 (62%; Table 1.1a). I removed from the experiment three clusters in which owls nested; cattle trampled two clusters prior to nest initiation and one cluster had been placed on an unacceptably steep slope according to the cluster deployment protocol described earlier. Removing these three clusters from the analysis does not change the conclusion of the chamber choice experiment. Of the remaining 31 clusters, nesting owls occupied 18 (58.1%). Of these 18 pairs of owls, 16 used the large chamber, one pair used the medium chamber, and one pair used the small chamber. This distribution of chamber use differs significantly from uniform (χ2 = 25.0, 2 df, P < 0.001). In 1998, I deployed 15 additional clusters of three artificial burrows (N = 5 in Kuna Butte and N = 10 in Grand View) but had to remove three of the original 34 clusters because of land development (Table 1.1b). This resulted in 46 clusters of three available in 1998. Of these, burrowing owls nested in 28 (61%; Table 1.1b). I removed from the experiment two clusters in which owls nested because cattle damaged them. Nesting owls occupied 26 (59.1%) of the remaining 44 clusters. These 26 pairs nested in large (N = 15), medium (N = 6), and small (N = 5) chambers. This distribution of chamber selection also differs significantly from uniform (χ2 = 7.0, 2 df, P < 0.03).
26 Table 1.1a. Occupancy (1997) of artificial burrow clusters of three placed around 1995 or 1996 natural nest burrows. Patterns of chamber size selection, clutch size, and number of fledglings are indicated for each cluster. An asterisk (*) indicates the minimum number of eggs as some nests were not checked until hatching began. Nest Name
Occupied?
Chamber Size Used
# Eggs Laid (# Fledgeda)
Kuna Butte Study Area Sewage Pond #1 Sewage Pond #2 Kuna Butte #1b Kuna Butte #5 Kuna Butte #7 Effluent Field North #1 Effluent Field South #1 Kuna Cave #2 Kuna Cave #3 Swan Falls #3 Swan Falls #4 Prison #1 J. Hayes #1 J. Hayes #2 J. Hayes #3 B. Stewart #1 B. Stewart #2
Yes Yes Yes No No Yes Yes No No No Yes Yes No No No Yes Yes
Large Medium Large Large Large Large Large Mediumc Smalld
11* (8) 9* (3)b 8 (3) 9 (8) 9 (8) 11 (10) 7 (7) 9 (8) 8 (4)
Grand View Study Area Trailer #1 Trailer #2 Trailer #4 Well #1 Well #2 Baha #1 Substation East #1 Trailer View #1 Grand View #2 Grand View #3 Grand View #19 Substation South #1 Substation South #2 Substation South #4
Yes Yes Yes No Yes Yes Yes Yes No No Yes Yes Yes No
Large Large Large Large Small Large Medium Large Large Large -
10 (5) 10* (5) 12* (8) 11 (5) 9* (4) 8* (8) 8* (7) 10 (3) 10 (7) 11* (6) -
27 Table 1.1a. Continued. Nest Name
Substation South #5 Substation South #6 Substation South #7 a
Occupied?
Chamber Size Used
# Eggs Laid (# Fledged)
Yes Yes No
Large Large -
12* (6) 9 (4) -
juvenile burrowing owls considered fledged at 28 d old. six eggs did not hatch. c cattle trampled large chamber, causing it to collapse prior to nest-site selection. d cattle trampled large chamber and tunnel to medium chamber, causing them to collapse prior to nest-site selection. b
28 Table 1.1b. Occupancy (1998) of artificial burrow clusters of three placed around natural burrows in which burrowing owls nested between 1995 – 1997. Patterns of chamber size selection, clutch size, and number of fledglings are indicated for each cluster. Nest Name
Occupied?
Chamber Size Used
# Eggs Laid (# Fledgeda)
Kuna Butte Study Area Kuna Butte #1b Kuna Butte #5 Kuna Butte #7 Effluent Field North #1 Effluent Field North #3 Effluent Field South #1 Kuna Cave #2 Kuna Cave #3 Swan Falls #3 Swan Falls #4 Swan Falls #6 Swan Falls #7 J. Hayes #1 J. Hayes #2 J. Hayes #3 B. Stewart #1 B. Stewart #2 B. Stewart #3 Poen #1
No No No Yes Yes Yes No No No Yes Yes Yes Yes Yes No Yes Yes Yes No
Smallb Large Small Large Small Medium Large Medium Smalld Large Large -
11 (0)c 10 (4) 9 (3) 10 (3) 9 (5) 10 (0) 9 (6) 9 (1) 11 (3) 11 (0) 12 (5) -
Grand View Study Area Trailer #1 Trailer #2 Trailer #4 Trailer #5 Well #1 Well #2 Baha #1 Baha #5 Substation East #1 Substation East #2 Trailer View #1 Trailer View #4 Grand View #2 Grand View #3
Yes Yes Yes Yes Yes No Yes No Yes No Yes No Yes No
Large Medium Medium Large Large Small Large Large Large -
10 (5) 7 (0)e 11 (0) 16 (6)f 10 (6) 10 (3) 11 (3) 10 (1) 7 (2)g -
29 Table 1.1b. Continued. Nest Name
Grand View #19 Substation South #1 Substation South #2 Substation South #4 Substation South #5 Substation South #6 Substation South #7 97-1 398-1 398-2 398-3 Backyard #2 Dirtmound #1 a
Occupied?
Chamber Size Used
# Eggs Laid (# Fledgeda)
Yes No Yes Yes Yes Yes Yes No Yes Yes No No No
Large Large Small Small Large Medium Large Medium -
12 (0) 10 (1) 9 (1) 8 (0) 11 (0) 6 (1) 9 (3) 12 (0) -
juvenile burrowing owls considered fledged at 28 d old. large chamber collapsed prior to nest-site selection. c nest chamber destroyed by development prior to when juveniles emerged. d cattle trampled large chamber and tunnel to medium chamber, causing them to collapse prior to nest-site selection. e represents second nesting attempt by this pair; pair attempted first nest at Trailer #4. f number of eggs and young fledged includes first (10 and 0, respectively) and second (6 and 6, respectively) nesting attempts by the same pair at this nest site. g values represent second nesting attempt by this pair; pair attempted first nest at Grand View #19. b
30 In summary, there were 80 clusters of three available as nest sites in 1997 and 1998, of which nesting pairs occupied 49 (61%). Excluding the five clusters mentioned above, nesting burrowing owls occupied 44 of 75 (59%) clusters of three over two seasons of my chamber choice experiment. Thirty-one pairs nested in the largest chamber available, seven pairs nested in the medium chamber, and six pairs nested in the small chamber. As in each individual year, this overall distribution of use differs significantly from uniform (χ2 = 27.3, 2 df, P < 0.001), which indicates that burrowing owls preferred to nest in the largest of the chambers I provided. Despite this preference for large chambers, mean clutch size did not differ among chambers (F2, 45 = 1.53, P = 0.227 for 1997 – 1998 combined), although average clutch sizes were larger for owls using large chambers when compared to those in small or medium chambers (Fig. 1.3). On the other hand, mean number of fledglings differed by chamber size (F2, 45 = 4.14, P = 0.022 for 1997 – 1998 combined; Fig. 1.3). Significantly more juveniles fledged from nests in large chambers when compared with medium chambers (P < 0.05). Effects of Tunnel Diameter My second experiment assessed preference for nest tunnel diameter using clusters of two artificial burrows. In 1997, I deployed 24 clusters (10 in Kuna Butte; 14 in Grand View), of which nesting owls occupied 12 (50%; Table 1.2a). Eight pairs of burrowing owls nested in the burrow with the small-diameter tunnel, while four pairs used the burrow with the large-diameter tunnel. This distribution of use did not differ significantly from uniform (χ2 = 1.33, 1 df, P > 0.24).
31 16
A
Small Medium Large
14
No. of eggs
12 10 8 6 4 2 2
2
16
6
6
16
8
8
32
0 1997 B
1998
Combined
12
No. of fledglings
10
Small Medium Large
8 6 B
4
AB A
2 2
0
2
1997
16
6
6
1998
16
8
8
32
Combined
Year Figure 1.3. (A) Clutch size (mean ± SE) for nests in small, medium, and large chambers were not significantly different in 1997, 1998, or when both years were combined. (B) Number of fledglings (mean ± SE) from nests in small, medium, and large chambers. Means with the same letter do not differ significantly (P < 0.05). If no letters appear, then overall ANOVA was judged not significant at α = 0.025 using Bonferroni correction. Sample sizes are indicated within or above bars.
32 Table 1.2a. Occupancy (1997) of artificial burrow clusters of two placed in suitable burrowing owl habitat and patterns of tunnel-diameter selection for each occupied nest burrow. Nest Name
Occupied?
Tunnel Diameter Used
# Eggs Laid (# Fledgeda)
Kuna Butte Study Area Kuna Butte Gravel #1 Kuna Butte Gravel #2 Effluent Field North #2 Effluent Field South #2 Kuna Cave Satellite #1 Kuna Cave Satellite #2 Kuna Butte Ag #1 Swan Falls Satellite #5 Prison #2 Prison #3
Yes No Yes Yes Yes No Yes No Yes No
10 cm 10 cm 15 cm 10 cm 15 cm 15 cm -
9 (4) 9 (9) 10 (7) 8 (7) 10 (8) 9 (2)b -
Grand View Study Area Powerline #1 Powerline #2 Powerline #3 Powerline #4 Powerline #5 Substation Southeast #1 Baha Pole #19 Trailer View #2 Trailer View #3 Well #3 Baha #2 Baha #3 Baha #4 Backyard #1
No Yes Yes No Yes No Yes No No Yes No Yes No No
10 cm 10 cm 10 cm 10 cm 15 cm 10 cm -
9 (2) 11 (2) 8 (2) 9 (7) 11 (11) 10 (6) -
a
juvenile burrowing owls considered fledged at 28 d old. red-tailed hawk (Buteo jamaicensis) killed adult male two days after last egg hatched; two young survived to fledge out of nine eggs laid.
b
33 Table 1.2b. Occupancy (1998) of artificial burrow clusters of two placed in suitable burrowing owl habitat and patterns of tunnel-diameter selection for each occupied nest burrow. Nest Name
Occupied?
Tunnel Diameter Used
# Eggs Laid (# Fledgeda)
Kuna Butte Study Area Kuna Butte Gravel #1 Kuna Butte Gravel #2 Junkyard #1 Kuna Butte #3b Kuna Butte #6b Honeybee #1 Honeybee #2 Effluent Field North #2 Effluent Field South #2 Effluent Field South #3 Kuna Cave Satellite #1 Kuna Cave Satellite #2 Kuna Cave Ag #1 Kuna Cave Ag #2 Kuna Butte Ag #1 Kuna Butte Ag #2 Kuna Butte Ag #3 Swan Falls Ag #1 Swan Falls Ag #2 Swan Falls Satellite #5
No No No Yes Yes No No Yes Yes Yes Yes No No Yes Yes Yes No Yes Yes Yes
15 cm 10 cm 10 cm 15 cm 15 cmc 10 cm 10 cm 10 cm 15 cm 10 cm 15 cm 15 cm
9 (2) 10 (5) 8 (2) 11 (5) 9 (1) 18 (6)d 9 (4) 8 (2) 8 (6) 11 (5) 8 (4) 9 (7)
Grand View Study Area Powerline #1 Powerline #2 Powerline #3 Powerline #4 Powerline #5 Substation Southeast #1 Baha Pole #19 Trailer View #2 Trailer View #3 Well #3 Baha #2 Baha #3
Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes
10 cm 10 cm 10 cm 10 cm 10 cm 10 cm 15 cm 10 cm 10 cm 15 cm 10 cm
9 (1) 9 (0)e 9 (3) 17 (0)f 11 (1) 10 (3) 10 (1) 9 (0) 9 (4) 10 (4) 11 (1)
34 Table 1.2b. Continued. Nest Name
Baha #4 Baha #6 Baha #7 Highway #1 Highway #2 Coyote Den #1 Dirtmound #2 Dirtmound #3 Dirtmound #4 Dirtmound #5 Backyard #1 Backyard #3 Backyard #4 Backyard #5 Backyard #6 Backyard #7 a
Occupied?
Tunnel Diameter Used
# Eggs Laid (# Fledgeda)
Yes Yes No Yes No Yes Yes No Yes No Yes Yes No Yes Yes No
15 cm 10 cm 10 cm 10 cm 10 cm 15 cm 10 cm 10 cm 15 cm 10 cm -
10 (4) 7 (2) 9 (4) 7 (4) 10 (2) 11 (1) 9 (6) 10 (0) 5 (0)g 8 (2) -
juvenile burrowing owls considered fledged at 28 d old. cluster of two replaced artificial burrow system placed in 1995. c 10-cm tunnel was damaged prior to nest-site selection and was therefore unavailable. d number of eggs and young fledged includes first (11 and 0, respectively) and second (7 and 6, respectively) nesting attempts by the same pair at this nest site. e represents first nesting attempt for this pair; second nest attempted at Coyote Den #1. f number of eggs and fledged includes first (11 and 0, respectively) and second (6 and 0, respectively) nesting attempts by the same pair at this nest site. g adults abandoned nest prior to completing clutch. b
35 In 1998, I deployed 26 additional clusters of two artificial burrows (N = 12 in Kuna Butte and N = 14 in Grand View) but had to remove two of the original 24 clusters (1997) because of land development (Table 1.2b). Of 48 possible clusters of two in 1998, nesting pairs of owls occupied 33 (69%; Table 1.2b). I removed one cluster from the analysis because one tunnel diameter was destroyed by farm equipment and was rendered unavailable to the owls just prior to nest-site selection. Of the remaining 47 clusters, nesting owls occupied 32 (68%). These 32 pairs used small-diameter (N = 22) tunnels and large-diameter (N = 10) tunnels. This distribution of tunnel selection does not differ significantly from uniform (χ2 = 3.8, 1 df, P < 0.06). For 1997 – 1998 combined, nesting burrowing owls occupied 44 of 71 (62%) clusters of two artificial burrows. Thirty pairs selected 10-cm diameter tunnels, while 14 pairs selected 15-cm diameter tunnels. This distribution of use differs significantly from uniform (χ2 = 5.1, 1 df, P < 0.025) and indicates that burrowing owls prefer to nest in burrows with smaller-diameter tunnels when given a choice. However, choice for smaller tunnels was not as consistently strong within years as was the apparent preference for chamber size. Although owls preferred to nest in smaller-diameter tunnels, clutch size (F1, 44 = 1.90, P = 0.175 for 1997 – 1998 combined) and number of fledglings/nest (F1, 44 = 4.85, P = 0.033 for 1997 – 1998 combined) did not differ between pairs nesting in burrows with small- or large-diameter tunnels (Fig. 1.4). Reuse of Nest Sites I also examined whether there was a tendency for owl pairs to nest at a specific site within an ABS cluster or whether nest choice was affected by chamber size or tunnel diameter. Very few individual owls nested in my study areas during both seasons, and
36 14
A
No. of eggs
12
10-cm 15-cm
10 8 6 4 2
8
0
4
1997
24
10
1998
32
14
Combined
14
B
No. of fledglings
12
10-cm 15-cm
10 8 6 4 2 8
0
1997
4
24
10
1998
32
14
Combined
Year Figure 1.4. (A) Clutch size (mean ± SE) for nests in 19-L buckets with small (10 cm) and large (15 cm) diameter tunnels. (B) Number of fledglings (mean ± SE) from nests with small and large diameter tunnels. Number of eggs and fledglings per nest did not differ between tunnel diameters within years or when years were pooled. Sample sizes are indicated within bars.
37 only males returned to their previous nest-site (i.e., breeding dispersal was female biased). Therefore, I analyzed site reoccupancy in 1998 and not reoccupancy of sites by specific birds. If in 1998 an occupied site within a cluster was the same as in 1997 (despite rearrangement of ABSs), I considered the owls to have selected an exact site rather than a specific ABS combination (chamber or tunnel type). In contrast, if owls chose the same ABS type in 1998 as the 1997 pair, even though it was relocated, then I considered owls to be selecting preferred ABS configurations rather than a specific site within a cluster. Of 21 occupied clusters of three in 1997, I removed three clusters from the ground prior to the breeding season because nearby land development either degraded surrounding habitat or denied access to the cluster. Another three clusters remained intact, but not all three chambers within a cluster were accessible in 1997 and/or 1998 because cattle trampled the chambers or tunnels. Of the 15 clusters of three remaining, 12 (80%) were occupied again in 1998 (Table 1.3). Of these, only one pair (8.3%) used the exact location as the 1997 pair (switched from large to small chamber; EFS #1, Table 1.3). Nine times owls used the same chamber size occupied by nesting owls in the previous year despite the rearrangement of the three chamber sizes. This distribution of chamber reuse (whether or not the same chamber size was used in both years) differs significantly from expected (χ2 = 8.41, 1 df, P < 0.005) and indicates that chamber size influenced nest-site selection more than site within a cluster of three ABSs. In 1998, two color-banded males (Trailer #1 and EFS #1, Table 1.3) returned to the same cluster they used in 1997. One of these males (Trailer #1) used the large chamber for nesting during both seasons despite the rearrangement procedure, whereas the other male (EFS #1) used the same location within its cluster but switched chamber
38 Table 1.3. Patterns of site reuse and chamber choice within clusters of three ABSs that owls used for nesting in 1997 and 1998. I identified males based on unique color-band combinations or inferred identity from unique behavior patterns when I could not capture them. Nest Name
1997 Chamber
1998 Chamber
Same Site within Cluster?
Same Male Each Year?
EFS #1
Large
Small
Yes
Yes
Swan Falls #4
Large
Large
No
Unknown
Substation #2
Large
Large
No
Suspected
Substation #5
Large
Small
No
Unknown
Substation #6
Large
Large
No
Suspected
Grand View #19
Large
Large
No
Unknown
Trailer View #1
Medium
Large
No
Unknown
Baha #1
Small
Small
No
Suspected
Substation East #1
Large
Large
No
Unknown
Trailer #1
Large
Large
No
Yes
Trailer #2a
Large
Medium
No
Unknown
Trailer #4
Large
Medium
No
Unknown
Kuna Butte Study Area
Grand View Study Area
a
pair from Trailer #4 re-nested here after failing in their first attempt in 1998.
39 types (large chamber in 1997, small chamber in 1998). Based on their behavior patterns (diving toward observer, allowing close approach, flushing to specific perches and locations), I suspected three additional unbanded males (Substation South #2, Substation South #6 and Baha #1, Table 1.3) as being males that returned in 1998 to the same cluster of three they had used in 1997. Each of these birds used the same chamber as they had used in the previous year (large, large, and small, respectively). In 1998, no females returned to the same cluster in which they nested during 1997, which is typical behavior of females in these study areas (see Belthoff and King 1997, Belthoff and Smith 1998, 1999). Of 12 clusters of two owls used as nest-sites in 1997, I could analyze 11 for site reuse in 1998. I had to remove one cluster from the ground prior to the breeding season because nearby land development denied access to the cluster. Owls reoccupied 10 of 11 (91%) clusters of two in 1998 (Table 1.4). Of these, three owl pairs (30%) used the same location as the 1997 pair, whereas seven pairs used the same tunnel diameter as the 1997 pair. This distribution of site reuse does not differ significantly from uniform (χ2 = 1.7, 1 df, P > 0.21), suggesting that owls neither selected for a site within a cluster nor "followed" a particular tunnel diameter. However, six of the seven clusters of two in which owls used the same tunnel diameter each year used 10-cm diameter tunnels in both seasons (see EFS #2 for exception, Table 1.4). Additionally, two of the three pairs that used the same location within a cluster as 1997 pairs (i.e., used tunnel diameters different than in 1997) used 10-cm diameter tunnels in 1998 rather than 15-cm diameter tunnels as the 1997 pair had (Kuna Butte Ag #1 and Well #3, Table 1.4).
40 Table 1.4. Patterns of site reuse and tunnel-diameter choice within clusters of two ABSs that were used for nesting in 1997 and 1998. I identified males based on unique colorband combinations or inferred identity from unique behavior patterns when I could not capture them. Nest Name
1997 Tunnel
1998 Tunnel
Same Site within Cluster?
Same Male Each Year?
EFN #2
10 cm
10 cm
No
Yes
EFS #2
15 cm
15 cm
No
Yes
Kuna Cave Satellite #1
10 cm
10 cm
No
Suspected
Kuna Butte Ag #1
15 cm
10 cm
Yes
Yes
Powerline #2
10 cm
10 cm
No
No
Powerline #3
10 cm
10 cm
No
Unknown
Powerline #5
10 cm
10 cm
No
Unknown
Baha Pole #19
10 cm
15 cm
Yes
No
Well #3
15 cm
10 cm
Yes
Unknown
Baha #3
10 cm
10 cm
No
No
Kuna Butte Study Area
Grand View Study Area
41 In 1998, three color-banded males returned to the same cluster they used in 1997. Two males (EFN #2 and EFS #2, Table 1.4) used the same diameter tunnel for nesting during both seasons (10-cm and 15-cm diameter tunnel, respectively) despite the rearrangement, whereas the third male (Kuna Butte Ag #1, Table 1.4) used the exact location within its cluster in 1998 as in 1997 (15-cm tunnel in 1997, 10-cm tunnel in 1998). Based on behavior patterns (diving toward observer, allowing close approach, flushing to specific perches and locations), I suspected one additional unbanded male (Kuna Cave Satellite #1, Table 1.4) as being a male that returned in 1998 to the same cluster of two he had used in 1997. This male used the same tunnel diameter (10-cm) in 1998 as in the previous year. In 1998, no females returned to the same cluster of two they used during 1997. In summary, owls reoccupied 12 of 15 (80%) clusters of three in 1998. Of these, only one pair returned to the same site within a cluster as the 1997 pair, whereas 11 pairs returned to a different site within a cluster as the 1997 pair. This distribution of reuse is significantly different from random and suggests that chamber size influenced nest-site selection more than sites within a cluster of three. Owl pairs (never the same pair as in 1997) reoccupied 10 of 11 (91%) clusters of two in 1998. Of these, three owl pairs used the exact location within the cluster as the 1997 pair, whereas seven pairs returned to the other site within the cluster. This distribution of site reuse does not differ significantly from uniform. Overall, there was no information in either experiment to suggest there was selection for a site rather than a chamber size or tunnel diameter.
42 Discussion Chamber Choice Experiment Large-chambered cavities provide more living space and may reduce negative effects of crowding, which may include increased levels of stress, intraspecific competition, and parasitism. Eastern screech-owls (Otus asio) in central Kentucky selected nest cavities of certain depths, avoiding extremely deep or shallow cavities (Belthoff and Ritchison 1990). Cavities of moderate depth may allow adult screech-owls easier access to young or quicker exit of the cavity to avoid an approaching predator, and may prevent predators from reaching young or eggs through the cavity entrance (Belthoff and Ritchison 1990). Similarly, great tits (Parus major), blue tits (P. caeruleus), and coal tits (P. ater) selected nest-boxes with deep cavities (19 cm) over ones with shallow cavities (9 cm; Summers and Taylor 1996). These species may prefer deeper boxes because they provide a better thermal environment or because the nest contents are not as accessible to predators (Summers and Taylor 1996). When nesting communally, prairie voles (Microtus ochrogaster) prefer nest chambers that are 49% larger than chambers that accommodate simple male-female pairs (Mankin and Getz 1994). Effects of crowding, and many other factors, influence the life history strategies (i.e., mate choice, colonial living, nest-site selection, nesting chronology; Møller l990) of a species and can result in a cost of reproductive efforts with regard to Darwinian fitness. Reproductive costs may be expressed as a reduction in survival for parents and/or offspring, future fecundity costs for parents and/or offspring, and costs in terms of timing of reproduction (Nur 1988, Møller 1993, Christe et al. 1996). Therefore, nest-site selection in cavity nesting species
43 is likely influenced by factors that maximize their fitness, and important factors are expected to vary from species to species. The first important finding in my study is that burrowing owls nested in the largest artificial burrow chambers significantly more often than in the two smaller configurations. The fact that owls prefer to nest in the larger chambers is consistent with the Overcrowded Hypothesis. Therefore, it appears that chamber size is an important criterion in nest-site selection by burrowing owls when several options are available to them. Moreover, this finding contradicts the suggestion by Collins and Landry (1977) that chamber size is relatively unimportant when deploying ABSs. In acorn woodpeckers (Melanerpes formicivorous), a cavity nesting species, only one of five nest characteristics perceived as being preferred by the woodpeckers was associated with increased levels of fitness (Hooge et al. 1999). Acorn woodpeckers, as well as other primary and secondary cavity nesters (e.g., Brawn 1988, Li and Martin 1991), exhibit several nest-site preferences, but only a limited number of those are associated with increased levels of nest success within their respective populations. Therefore, Hooge et al. (1999) conclude that demographic and ecological constraints play a larger role than nest-site quality in nest-site selection in acorn woodpeckers. Although tested in separate experiments, burrowing owls in my study preferred nests with large chambers and 10-cm diameter tunnels; only large chambers were associated with increased productivity. Based on chamber choices, reuse patterns, and estimates of productivity in the clusters of three in my study, it seems large chambers offer more benefits than other chamber sizes and particular sites within a cluster. This suggests certain nest-chamber qualities may be important in nest-site selection by burrowing owls.
44 Although my results are consistent with the Overcrowded Hypothesis, there are other potential explanations for choice for larger nest chambers. It might be that microclimates (temperature regimes, gaseous environment) in large chambers are more suitable for incubation or brood rearing. Choosing an area with a stable microclimate (e.g., dense vegetation, cavity) can conserve considerable amounts of energy, which individuals can then use for reproduction, resource defense, and social activities (Walsberg 1985). Kendeigh (1961) found that a house sparrow (Passer domesticus) roosting in a nest box could save as much as 13.4% of the total energy allocated for roosting if nightly ambient temperatures were to drop to -30°C. Similarly, while an acorn woodpecker could save 9.0% of its allotted roosting energy if it roosted alone in a cavity rather than in the open, it could save 17.0% of its energy by roosting with three other conspecifics (du Plessis et al. 1994). Gaining thermal advantages through nest placement has been found or suggested as an important factor in nest-site selection in many aboveground (Kern and van Riper III 1984, Bekoff et al. 1987, van Riper III et al. 1993, Zwartjes and Nordell 1998, Hooge et al. 1999) and underground nesting bird species (Ellis 1982, Birchard et al. 1984). A favorable nest microclimate may increase reproductive success by decreasing the energetic costs of incubation (White and Kinney 1974), preventing death by exposure, or increasing the growth rate of juveniles (Quinney et al. 1986). Also, although dimensions of natural underground chambers and tunnels vary depending on their original excavator (Ellis 1982, Birchard et al. 1984, Haug et al. 1993), underground burrow systems offer fairly stable environments that protect their inhabitants from temperature extremes (White et al. 1978, Ellis 1982). The largest chamber in my experiment, in addition to reducing potential effects of crowding, may
45 have offered a more desirable microclimate than other chambers. Perhaps humidity levels were higher in large chambers because of increased space for nest material (dirt, manure), or carbon dioxide concentrations were lower because of increased amounts of atmospheric volume within the chamber. Tunnel Choice Experiment My study also found that tunnel diameter affected nest-site selection; owl pairs nested in artificial burrow systems with 10-cm diameter tunnels significantly more often than expected by chance. The fact that owls preferred to nest in ABSs with smallerdiameter tunnels is not consistent with the Overcrowded Hypothesis. While there are advantages to larger-diameter tunnels, as my hypothesis outlined, burrowing owls may prefer smaller-diameter tunnels because they deter larger ground-dwelling predators, such as American badgers or striped skunks (Mephitis mephitis), both of which occur in my study areas. Badgers can account for nearly 90% of known depredation of burrowing owl nests (Green 1983, Haug et al. 1993). I often found signs of badger and coyote (Canis latrans) visits to nests in ABSs (tunnels excavated along-side ABS tunnels, canine holes at the end of an ABS tunnel), but could not determine how successful the predators were at capturing juvenile owls before they entered the ABS. Selection of 10-cm diameter tunnels by burrowing owls could represent a trade-off of available space with added protection against nest predation. That is, larger-diameter tunnels (15 cm) offer more space for juvenile and adult owls and allow more than one owl under the threat of predation to enter a burrow at a time, but at the same time larger tunnels facilitate the potential entry of larger predators. Therefore, my results are consistent with the notion
46 that burrowing owls prefer the potential added safety of smaller-diameter tunnels over additional tunnel space. Alternatively, microclimates in smaller-diameter tunnels may be more suitable for incubation, brood rearing, or gas exchange with the outside environment. As previously mentioned, a favorable microclimate would offer several advantages to individuals or groups within an ABS, most of which influence survival and fecundity. However, neither mean clutch size nor number of fledglings varied significantly by tunnel diameter. Because 10-cm and 15-cm tunnels did not differ significantly in either measure of reproductive output, one might assume that other benefits are derived from the preference for 10-cm diameter tunnels. Perhaps smaller-diameter tunnels also reduce light levels within the nest chamber, or they may decrease the likelihood of a flooding event. Follow-up research could help distinguish among the alternative hypotheses for preference for smaller tunnels. Nonetheless, the lack of understanding about why the owls prefer such configurations probably should not deter the use of such tunnels for management or research purposes. Reuse of Nest Sites It is not uncommon for burrowing owls to reuse burrows from year to year (Haug et al. 1993), but rate of reuse seems to vary among populations. For example, Martin (1973) observed that each of 15 pairs of burrowing owls in New Mexico used burrows that owls occupied in previous years. Plumpton and Lutz (1993) found that owls reused four of 20 (20%) nesting burrows during the second year of their study in Colorado. Finally, Rich (1984) determined that of 242 nest sites located between 1976 – 1983 in
47 south-central Idaho, 115 (39.4%) also were occupied in at least one subsequent year, although this latter study did not confirm breeding at all sites. My research in Kuna Butte and Grand View documented the extent to which owls reused nest sites between 1997 and 1998. In 1998, burrowing owls reused 25 of 29 (86%) nest sites containing clusters of ABSs. This rate of ABS reuse is greater than rates of reuse for natural burrows in my study areas (see Belthoff and King 1997, Belthoff and Smith 1998) and surrounding areas (Rich 1984, Lehman et al. 1998), illustrating what may be an important opportunity for management of burrowing owl populations. Over just a few years in my study areas, agriculture, fire rehabilitation, and various other activities (including trampling by grazing cattle) destroyed many nest burrows or potential nest burrows (both natural and artificial). Given what appears to be a strong tradition of burrow reuse in this species, deploying artificial burrow systems in suitable habitat and preventing destruction of commonly used burrows, particularly those that successfully produced young (see Belthoff and King 1997; Belthoff and Smith 1998), likely could be an important first step to avoid population declines. Of course, such management practices would be most critical in areas where burrows are limited in availability, which is not necessarily the case in and around my study areas (pers. obs.), although this issue deserves further study. Alternatively, areas devoid of burrows, or in which burrows must be destroyed for development or other management practices, could be supplemented with artificial burrows to enhance habitat for burrowing owls. This study has determined a configuration of artificial burrows that burrowing owls prefer over configurations used in previous studies, and such burrows can now be deployed for various management purposes.
48 Conclusions My study confirms that chamber and tunnel dimensions are important features affecting nest-site selection in burrowing owls. When given a choice, adult owls preferred the largest chamber available and selected 10-cm diameter tunnels for their artificial nest burrow configurations. Preferences by adult owls in my chamber size experiment were consistent with the predictions of the Overcrowded Hypothesis, whereas the results from the tunnel diameter experiment were consistent with a potential trade-off between available space and predator deterrence: owls may prefer large chambers to benefit from the available space within a nest chamber and smaller tunnel diameters to exclude a class of important nest predators. Burrowing owls also showed high affinity to ABS clusters between years; in 1998, owls reoccupied nearly 85% of previously used clusters. Despite the overwhelming choice for large chambers and 10-cm diameter tunnels, my design did not allow me to confirm that owls would have selected an ABS with a large chamber and 10-cm diameter tunnel. My data suggest that ABSs with these dimensions would be preferred by burrowing owls, but neither of my experiments incorporated this specific ABS configuration. Based on the decisive outcome of each ABS experiment (chamber and tunnel choice), it is reasonable to expect that owls would select ABSs composed of large chambers and 10-cm diameter tunnels more often than any of the other combinations I provided. Finally, it is doubtful that only one factor influences burrowing owls to select large chambers and 10-cm diameter tunnels preferentially. Decisions concerning nest burrow dimensions may be influenced by a complex interaction of ecological, social,
49 behavioral, and physiological factors. However, based on my ABS experiments, I suggest that ample space within a large chamber and increased predator deterrence derived from using 10-cm diameter tunnels are among the features important in burrowing owl nest-site selection.
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50 Brawn, J. 1988. Selectivity and ecological consequences of cavity nesters using natural vs. artificial nest sites. Auk 105:789-791. Christe, P., H. Richner, and A. Oppliger. 1996. Of great tits and fleas: sleep baby sleep. Anim. Behav. 52:1087-1092. Collins, C.T., and R.E. Landry. 1977. Artificial nest burrows for burrowing owls. N. Amer. Bird Bander 2:151-154. Coulombe, H.N. 1971. Behavior and population ecology of the burrowing owl, Athene cunicularia, in the Imperial Valley of California. Condor 73:162-176. Delevoryas, P. 1997. Relocation of burrowing owls during courtship period. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:138-144. du Plessis, M.A., W.W. Weathers, and W.D. Koenig. 1994. Energetic benefits of communal roosting by acorn woodpeckers during the nonbreeding season. Condor 96:631-637. Dyer, O. 1991. Reintroduction of burrowing owls (Athene cunicularia) to the South Okanagan Valley, British Columbia (1983-1988). Provincial Mus. of Alberta Nat. Hist. Occasional Paper No. 15. Ellis, J.H. 1982. The thermal nest environment and parental behavior of a burrowing bird, the bank swallow. Condor 84:441-443. Feeney, L.R. 1997. Burrowing owl site tenacity associated with relocation efforts. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:132-137. Gleason, R.S. 1978. Aspects of the breeding biology of burrowing owls in southeastern Idaho. Unpubl. M.S. Thesis, Univ. of Idaho, Moscow. Green, G.A. 1983. Ecology of breeding burrowing owls in the Columbia basin, Oregon. Unpubl. M.S. Thesis, Oregon State Univ., Corvallis. Harris, R.D., and L. Feeney. 1990. Restoration of habitat for burrowing owls (Athene cunicularia). Pp. 251-259. In: H.G. Hughes and T.M. Bonnicksen, eds. Restoration ’89: the new management challenge. Proceedings of the society for ecological restoration first annual meeting. The Univ. of Wisconsin Arboretum, Madison.
51 Haug, E.A., D. Hjertaas, S. Brechtel, K. De Smet, O. Dyer, G. Holroyd, P. James, and J. Schmutz. 1992. National recovery plan for the burrowing owl. A report prepared for the Committee for the Recovery of Nationally Endangered Wildlife. Canadian Wildlife Federation, Ottawa. 49 pp. Haug, E.A., and A.B. Didiuk. 1993. Use of recorded calls to detect burrowing owls. J. Field Ornithol. 64:188-194. Haug, E.A., B.A. Millsap, and M.S. Martell. 1993. Burrowing owl (Speotyto cunicularia). In: A. Poole and F. Gill, eds. The Birds of North America, No. 61. The Academy of Natural Sciences, Philadelphia; The American Ornithologists' Union, Washington, D.C. 20 pp. Henny, C.J., and L.J. Blus. 1981. Artificial burrows provide new insight into burrowing owl nesting biology. Raptor Res. 15:82-85. Hironaka, M., M.A. Fosberg, and A.H. Winward. 1983. Sagebrush-grass habitat types of southern Idaho. Forest, Wildl. and Range Expt. Stat., Univ. of Idaho, Moscow, Bull. No. 35. 44 pp. Hooge, P.N., M.T. Stanback, and W.D. Koenig. 1999. Nest-site selection in the acorn woodpecker. Auk 116:45-54. James, P.C., and R.H.M. Espie. 1997. Current status of the burrowing owl in North America: an agency survey. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:3-5. Kendeigh, S.C. 1961. Energy of birds conserved by roosting in cavities. Wilson Bull. 73:140-147. Kern, M., and C. van Riper III. 1984. Altitudinal variations in nests of the Hawaiian honeycreeper Hemignathus virens. Condor 86:443-453. King, R.A. 1996. Post-fledging dispersal and behavioral ecology of burrowing owls in southwestern Idaho. Unpubl. M.S. Thesis, Boise State Univ., Idaho. Landry, R.E. 1979. Growth and development of the burrowing owl. Unpubl. M.S. Thesis, California State Univ., Long Beach. Lehman, R.N., L.B. Carpenter, K. Steenhof, and M.N. Kochert. 1998. Assessing relative abundance and reproductive success of shrubsteppe raptors. J. Field Ornithol. 69:244-256. Leptich, D.J. 1994. Agricultural development and its influence on raptors in southern Idaho. Northwest Sci. 68:167-171.
52 Li, P., and T.E. Martin. 1991. Nest-site selection and nesting success of cavity-nesting birds in high elevation forest drainages. Auk 108:405-418. Mankin, P.C., and L.L. Getz. 1994. Burrow morphology as related to social organization of Microtus ochrogaster. J. Mammal. 75:492-499. Martell, M.S. 1990. Reintroduction of burrowing owls into Minnesota: a feasibility study. Unpubl. M.S. Thesis, Univ. of Minnesota, Minneapolis. Martin, D.J. 1973. Selected aspects of burrowing owl ecology and behavior. Condor 75: 446-456. Millsap, B.A., and C. Bear. 1997. Territory fidelity, mate fidelity, and dispersal in an urban-nesting population of Florida burrowing owls. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:91-98. Møller, A.P. 1990. Effects of parasitism by the haematophagous mite on reproduction in the barn swallow. Ecology 71:2345-2357. Møller, A.P. 1993. Ectoparasites increase the cost of reproduction in their hosts. J. Anim. Ecol. 62:309-322. NOAA (National Oceanic and Atmospheric Administration). 1985. Climates of the States: 1951-1980. Vol. 1, 3rd ed. 758 pp. Nur, N. 1988. The cost of reproduction in birds: an examination of the evidence. Ardea 76:155-168. Olenick, B.E. 1990. Breeding biology of burrowing owls using artificial nest burrows in southeastern Idaho. Unpubl. M.S. Thesis, Idaho State Univ., Pocatello. Plumpton, D.L., and R.S. Lutz. 1993. Nesting habitat use by burrowing owls in Colorado. J. Raptor Res. 27:175-179. Quinney, T.E., D.J.T. Hussel, and C.D. Ankney. 1986. Sources of variation in growth of tree swallows. Auk 103:389-400. Rich, T. 1984. Monitoring burrowing owl populations: implications of burrow re-use. Wildl. Soc. Bull. 12:178-180. Rich, T. 1986. Habitat and nest-site selection by burrowing owls in the sagebrush steppe of Idaho. J. Wildl. Manage. 50:548-555.
53 Summers, R.W., and W.G. Taylor. 1996. Use by tits of nest boxes of different designs in pinewoods. Bird Study 43:138-141. Thomsen, L. 1971. Behavior and ecology of burrowing owls on the Oakland municipal airport. Condor 73:177-192. Trulio, L.A. 1995. Passive relocation: a method to preserve burrowing owls on disturbed sites. J. Field Ornithol. 66:99-106. Trulio, L.A. 1997. Strategies for protecting western burrowing owls (Speotyto cunicularia hypugaea) from human activities. Pp. 461-465. In: J.R. Duncan, D.H. Johnson and T.H. Nicholls, eds. Biology and conservation of owls of the northern hemisphere: proceedings of the second international owl symposium. U.S. Dept. of Agric. Gen. Tech. Rept. NC-190. Turner, J. 1985. The burrowing owl transplant program in the South Okanagan Valley of British Columbia. Unpubl. Rept. for JT Biotech Assoc., Inc., Princeton, British Columbia. 63 pp. van Riper III, C., M.D. Kern, and M.K. Sogge. 1993. Changing nest placement of Hawaiian common amakihi during the breeding cycle. Wilson Bull. 105:436-447. Walsberg, G.E. 1985. Physiological consequences of microhabitat selection. Pp. 389413. In: M.L. Cody, ed. Habitat selection in birds. Academic Press, New York. White, F.N., and J.L. Kinney. 1974. Avian incubation. Science 186:107-115. White, F.N., G.A. Bartholomew, and J.L. Kinney. 1978. Physiological and ecological correlates of tunnel nesting in the European bee-eater, Merops apiaster. Physiol. Zool. 51:140-154. Zar, J.H. 1996. Biostatistical analysis. 3rd edition. Prentice-Hall, Inc., Upper Saddle River, New Jersey. Zarn, M. 1974. Burrowing owl, Athene cunicularia hypugaea. Report No. 11, Habitat management series for unique or endangered species. Bureau of Land Mgmt., Denver, Colorado. 25 pp. Zwartjes, P.W., and S.E. Nordell. 1998. Patterns of cavity-entrance orientation by gilded flickers (Colaptes chrysoides) in cardón cactus. Auk 115:119-126.
54
CHAPTER 2: ECTOPARASITES ON BURROWING OWLS IN SOUTHWESTERN IDAHO: ASSESSMENT OF POTENTIAL EFFECTS ON SITE REUSE AND JUVENILE GROWTH, HEALTH, AND SURVIVAL
Abstract I examined ectoparasites on burrowing owls (Athene cunicularia) in southwestern Idaho during the 1997 and 1998 breeding seasons. I found three species of flea (Pulex irritans, Aetheca wagneri, and Meringis hubbardi), one species of louse (Strigiphilus speotyti), and one species of carnid fly (Family: Carnidae). Ectoparasite loads per brood did not differ between years among three artificial burrow chamber sizes or between two artificial burrow tunnel diameters. Also, ectoparasite load at a nest-site in 1997 did not affect nest-site reoccupancy in 1998. Ectoparasite load did not affect growth of juveniles or the number of fledglings per nest. Finally, I detected no difference in hematocrit levels or rates of growth between owls with natural levels of ectoparasites and owls whose ectoparasites were removed. Although elevated levels of ectoparasites existed in many nests, burrowing owls seemed to overcome any negative effects of their presence except when their abundance was extreme. Perhaps burrowing owls have adopted several anti-parasite behaviors to reduce disadvantages imposed by ectoparasite accumulation within a nest burrow.
55 Introduction Parasitism, the acquisition of resources from other living organisms, is one of the most common forms of gaining resources (Price 1980). Parasitism influences life history strategies of a species (Møller l990) because the host incurs a loss of energy from the parasite's presence unless an individual is able to acquire sufficient resources to overcome the deficit. Reproduction in organisms also requires substantial amounts of energy for the parents (territorial defense, mate acquisition, egg/embryo development, feeding and protection of young) and offspring (growth and motor skill development). Breeding adults or growing juveniles, when parasitized, can suffer from loss of energy vital to their success or survival, and thus are negatively affected by parasitism (Møller et al. 1994, Oppliger et al. 1997). Energy lost through reproduction in addition to parasitism may reduce survival, add future fecundity costs for parents and/or offspring, and may increase costs in terms of timing of reproduction (Nur 1988, Møller 1993, Christe et al. 1996a, Tripet and Richner 1997). Numerous studies have investigated host/parasite interactions in an attempt to explain how ectoparasite infestation influences the hosts’ life history strategies. In some bird species, ectoparasites reduce clutch size, nestling body mass, and the number of young at hatching and fledging (Møller 1990, 1993, Richner et al. 1993, Møller et al. 1994, Christe et al. 1996a, Dufva and Allander 1996, McFadzen and Marzluff 1996), increase mortality (Møller 1990, 1993, Richner et al. 1993, McFadzen and Marzluff 1996), increase nestling begging rates (Christe et al. 1996a) and adult provisioning rates (Tripet and Richner 1997), reduce adult sleeping time (Christe et al. 1996b), and reduce nestling hematocrit levels (McFadzen and Marzluff 1996). In contrast, a few studies
56 found no effect of ectoparasitism on reproductive success or survival in adults or nestlings (e.g., Roby et al. 1992, Young 1993, Gebhardt-Henrich et al. 1998). If parasites negatively affect their hosts or their hosts' offspring then, as many of the above studies suggest, natural selection should promote patterns of anti-parasite behavior or physiological responses that reduce the parasites' impact (Clark 1991, Clayton 1991, Hart 1992, Christe et al. 1996a, 1996b, Heeb et al. 1996). One such behavior pattern that potentially could be modified is nest-site selection. Reuse of a previous nest could save birds both energy and time and thus potentially enhance reproductive output (Conrad and Robertson 1993, Gauthier and Thomas 1993). On the other hand, nest-site quality or suitability may decline with each successive use and make it more likely that individuals will move to a new site to nest (Møller 1989, Gowaty and Plissner 1997). For instance, nest-site quality may be reduced between years by the overwintering of certain ectoparasites in old nest material (Moss and Camin 1970, Brown and Brown 1986, Barclay 1988, Møller 1989, Thompson and Neill 1991, Oppliger et al. 1994, Johnson 1996) or suitability of an old site may decline if a nest predator revisits a site it raided in a previous breeding season (Nilsson et al. 1991, Sonerud 1993). Consequently, secondary cavity nesting species may prefer clean (previously unused or old nest removed) cavities for nesting and, if clean cavities are not available, will exhibit higher rates of breeding dispersal (Møller 1989, Thompson and Neill 1991, Gowaty and Plissner 1997). Underground burrow systems may be particularly susceptible to ectoparasite invasions because burrows offer moderate temperatures and high humidity (see Mayer 1955 and Kennerly 1964 in Butler and Roper 1996) along with decaying nest material
57 and available hosts, each of which likely favors ectoparasite survival and reproduction (Butler and Roper 1996). Burrowing owls (Athene cunicularia) nest in burrow systems and their nests harbor at least 39 different species of arthropods (21 families: Philips and Dindal 1977), some of which most likely parasitize adult owls and their young. Burrowing owl nests in southwestern Idaho exhibit various levels of ectoparasite infestation; fleas (Siphonaptera) and chewing lice (Mallophaga) are particularly common (J. Belthoff, pers. comm., pers. obs.). Some nests exhibit such high loads that over 100 fleas can be counted on the body of a bird from that nest. Such high levels of ectoparasitism may have negative impacts on juvenile behavior patterns (Clayton 1991, Christe et al. 1996a), body condition (Møller 1990, 1993, Richner et al. 1993, Møller et al. 1994, McFadzen and Marzluff 1996), and survival (Møller 1990, 1993, Richner et al. 1993, Møller et al. 1994, McFadzen and Marzluff 1996). Because burrowing owl populations are declining throughout much of their range, understanding the effects of ectoparasites on this species may be important in those regions where intense management (e.g., translocations, hacking) of burrowing owl populations is necessary. My study was designed to: (1) collect and identify ectoparasite species on burrowing owls in southwestern Idaho, (2) examine potential effects of ectoparasites on reproductive success of owl pairs, (3) determine relationships between ectoparasite loads and artificial burrow chamber and tunnel size (see Chapter One for details about artificial burrows), (4) investigate inter-year variation in ectoparasite loads at nest sites used in 1997 and 1998, and (5) examine potential effects of ectoparasites on growth of juveniles. To accomplish these tasks, I monitored nest success (i.e., clutch size, brood size, number of fledglings per breeding attempt) of all nests in artificial burrow systems (ABSs) and
58 indexed each nest's ectoparasite load at the time when I banded nestlings. I collected ectoparasites from randomly selected adult and juvenile owls and preserved them for later identification. Also, I randomly selected nests to participate in a separate experiment in which I eliminated ectoparasite loads in some nests (dusted), and allowed some nests to maintain their natural ectoparasite populations (infested). I then compared hematocrit levels and nestling body measurements (i.e., body mass and wing chord) to examine potential effects of ectoparasites on growth and survival in juvenile owls.
Study Area and Methods Study Areas I examined the diversity, abundance, and potential effects of ectoparasites on burrowing owls in two study areas in southwestern Idaho. The first was located approximately 3.2 km south of Kuna (43° 25' N, 116° 25' W) and 23 km north of the Snake River Canyon, in Ada County. Vegetation in this area is characterized by big sagebrush (Artemisia tridentata) shrubland and disturbed grasslands dominated by cheatgrass (Bromus tectorum) and tumble mustard (Sisymbrium altissimum). Surrounding areas contain irrigated agricultural fields (primarily alfalfa, mint, and sugar beets), scattered residential homes, and several large dairy farms. The topography is flat to rolling with elevations ranging from 841 m to 896 m. Rock outcrops and a few isolated buttes (e.g., Kuna Butte, elev. 986 m) exist in the region. Temperatures range from -20° to 45°C, and annual precipitation averages less than 20 cm (NOAA 1985). In this area, there is a relatively high density of burrows excavated by Townsend's ground squirrels (Spermophilus townsendii) and American badgers (Taxidea taxus); burrowing
59 owls regularly use burrows abandoned by badgers for nesting and shelter throughout the breeding season. The second study area was located approximately 8 km north-northeast of Grand View (43° 00' N, 116° 00' W), in Elmore County, Idaho, and adjacent to State Highway 67. This area is a mosaic of irrigated agriculture and disturbed grasslands. Elevations range from 853 m to 922 m. The area contains very few homes, several paved and dirt roads, and an electrical substation. The Snake River is located approximately 7 km south-southwest of this study area. Temperatures here range from -29° and 43°C, and precipitation averages 26 cm per year (NOAA 1985). Both study areas were once typical shrub-steppe communities dominated by large expanses of big sagebrush (Hironaka et al. 1983). Range fires and other disturbances have converted much of the shrublands to exotic annual communities dominated by cheatgrass and tumble mustard. In general, the Kuna Butte study area contains more native shrubland than the seemingly more disturbed Grand View study area (Belthoff and King 1997). Locating and Capturing Burrowing Owls All owls in this study nested in artificial burrow systems. Owls nested either in clusters of two ABSs, which offered two tunnel diameters (small and large) with a standard chamber size, or in clusters of three ABSs, which offered three chamber sizes (small, medium, and large) with a standard tunnel diameter (see Chapter One for details). Digging down to nest chambers allowed repeated access to nest substrates and easy capture of juvenile owls.
60 To capture adult owls I used Havahart® traps and noose rods as described in Belthoff et al. (1995) and King (1996). I also used one-way basket traps to capture adults as they departed artificial burrows. These traps consisted of a 0.5-m section of flexible plastic pipe (10-cm diameter), a small piece of transparent Plexiglas®, and an enclosure made of “chicken wire”. The Plexiglas® was fastened to one end of the pipe by a hinge from above. This end of the pipe inserted into the wire basket. The open end of the pipe inserted into artificial burrow tunnels when I needed to check the status of a nest in an ABS. Digging down to the artificial chamber caused any adults in the nest burrow to enter the basket, the hinged door closed behind them, and the owl became trapped. Measuring and Marking Owls Upon capture, I recorded each owl's mass (to the nearest gram), wing length, tarsus length, tail length, and length of exposed culmen (all to the nearest 0.5 mm). I classified adult owls as female if they had a well-developed brood patch. I classified the remainder of adults as males based on their lack of a brood patch, relatively lighter plumage, and behavior near nests (Thomsen 1971, Zarn 1974). I could not discern sex of young owls based on appearance or morphological measurements. I fitted owls with a United States Geological Survey aluminum leg band (size 4) and three plastic, colored leg bands (National Band and Tag Co., Newport, KY) for future identification. Ectoparasite Identification To catalogue ectoparasite species infesting burrowing owls in southwestern Idaho, I randomly selected a subset of individual owls (N = 11) and entire broods (N = 4) from birds I captured. I then removed ectoparasites from the bodies of these owls using surgical tweezers to grasp any visible parasites (most effective for lice) or dusting birds
61 with 5% Malathion powder (most efficient for flea collection). After dusting an individual (or brood), I placed the bird(s) in a plastic container for 10 min, which was sufficient time for the insecticide to operate on ectoparasites (pers. obs.). I preserved specimens of all ectoparasites in 70% alcohol for later identification. Levels of Infestation Upon capture, I assessed each burrowing owl's ectoparasite load. Before dusting each bird (see below), feathers on the back of the head were gently separated with a pencil to expose the skin. I then counted the number of ectoparasites by systematically moving from left to right on the head. To avoid duplicate counting, parasites that moved to any portion of the head not yet censused were subtracted from the count. Next, I examined feathers from the back, pelvic, scapular, axillary, and primary regions for ectoparasites or clinical signs of infestation. Infested birds can have denuded feather shafts, puncture holes in feather quills, and poor feather condition (Turner 1971, Clayton 1991). Finally, I noted the relative number of ectoparasites that crawled onto my hand during the ectoparasite inspection. I classified birds into three levels of infestation based on the observed variations in ectoparasite loads of all the above mentioned indices: level 1 (low; five or fewer individuals), level 2 (medium; more than five and up to 10 individuals), and level 3 (high; greater than 10 individuals). I then used these classification levels as an index of brood parasite load by assessing each individual's load within the brood, summing their classification levels, and then dividing by the number of owls in the brood to obtain an average classification level for the entire brood. I used brood ectoparasite loads to examine relationships between ectoparasite loads and
62 artificial burrow chamber (N = 41) and tunnel size (N = 40), and inter-year variation in ectoparasite loads at nest sites occupied in both seasons of my study (N = 22). Juvenile Growth and Body Condition To examine potential effects of ectoparasites on growth by juveniles, I performed an experiment with a random sample of nests (N = 5) in artificial burrow systems. At all chosen nests, I first assessed ectoparasite loads of juveniles (as described above) and nest substrates. To index a nest substrate's ectoparasite load, I placed my hand on the nest’s substrate for 30 sec and continually disturbed the substrate to simulate bird movement. I noted the relative number of ectoparasites on my hand and on the substrate and classified the nest as having a low, medium, or high infestation level based on the observed variations in ectoparasite loads. During my first visit to dusted nests (N = 2), I applied insecticide to the nest substrate and nestlings to eliminate ectoparasites. To control for disturbance, I treated all nest substrates and juveniles identically except I applied no insecticide to juveniles or nest substrates at infested nests (N = 3). I could not increase ectoparasite levels experimentally because burrowing owls are provided special status (i.e., species of special concern) in Idaho. It was unclear if increasing ectoparasite loads would have detrimental effects on nest success or survival in burrowing owls; therefore, I chose to remove effects of ectoparasites in some broods by dusting them with insecticide while allowing other nests to contend with their unaltered ectoparasite burden. Once ectoparasite manipulations were complete, I recorded each owl's mass (to the nearest gram) and wing length (to the nearest 0.5 mm). Additionally, I collected blood samples from all juvenile owls to obtain hematocrit levels for owls in each treatment group. I visited both dusted and infested nests on two occasions; the second
63 visit occurred five days after the initial visit. During each visit, I assessed ectoparasite loads, recorded measurements to assess growth, and acquired blood samples. Owl Monitoring I conducted follow-up visits to all nest burrows to determine if owl pairs successfully fledged young. For owls nesting in ABSs, I determined the number of juveniles fledged per nest. Successful nests from artificial burrow systems had at least one young owl survive to fledging age (> 28 d). Data Analyses Using two-factor analysis of variance (ANOVA), I examined effects of year and chamber size, and the effects of year and tunnel diameter, on ectoparasite load of each brood (range: 0.00 – 3.00). I performed these tests on brood averages of ectoparasite load because of the lack of independence of observations for juveniles in a brood. Chi-square approximation values provided by these tests were considered significant if P < 0.05. Multivariate analysis of variance (MANOVA) was used to examine relationships between relative number of ectoparasites on the owl's head and my hand as a function of tunnel diameter and chamber size. To test the null hypothesis that ectoparasite load of a nest-site in 1997 did not affect nest-site reuse in 1998, I used contingency table analysis (G-test) to determine the likelihood of owls in 1998 returning to the same tunnel/chamber given its 1997 ectoparasite load. I also used Spearman rank correlation analysis to assess the relationship between an ABS's ectoparasite load between years. I also performed multivariate analyses of covariance (MANCOVA) to examine effects of ectoparasites on growth in juvenile burrowing owls while using brood averages. Average age of nestlings (days) and brood size were used as covariates. I also used repeated measures analysis of
64 variance to examine differences in hematocrit levels and rates of growth between dusted and infested nests in my ectoparasite removal experiment. I analyzed brood averages for each nest in the experiment, and because two dependent variables (hematocrit level and rates of growth) were collected from the same experimental units and analyzed, I considered differences significant when P < 0.025 (Zar 1996). All means are given as x ± SE.
RESULTS Ectoparasite Identification I collected five species of ectoparasitic insects from burrowing owls (Table 2.1). Of three species of flea collected, Pulex irritans (Family: Pulicidae) was most prevalent (124 of 143; 86.7%) in both study areas. I found P. irritans on three adult owls and on three broods of owls containing six, 10, and 11 juveniles. I collected 15 (10.5% of total) Aetheca wagneri (Family: Ceratophyllidae), but I found this species on only one brood (N = 10 juveniles) of owls in the Kuna Butte study area. I collected four (2.8% of total) specimens of Meringis hubbardi (Family: Hystrichopsyllidae), with each of the four fleas coming from a different owl (three from adult owls and one from a brood of 10 juveniles). Meringis hubbardi was found in both study areas in very low frequencies. All of the lice I collected (N = 8, 100%) were Strigiphilus speotyti. I found two males, three females, and three immatures and collected this species from owls (N = 5) in both study areas. I also collected two specimens of a fly (Family: Carnidae) from one brood (N = 6 juveniles) of owls in Grand View (Table 2.1). Wing and setae characteristics
65 Table 2.1. Species and demographics of ectoparasites collected from burrowing owls in southwestern Idaho during 1997 and 1998. I collected ectoparasites from a subset of individual owls (N = 11) and entire broods (N = 4) that I captured. Species
Sex
Age
Number Collected
Male
Adult
33
Female
Adult
39
Male
Adult
1
Female
Adult
14
Meringis hubbardi
Male
Adult
1
Strigiphilus speotyti
Female
Adult
2
Male
Adult
20
Female
Adult
32
Male
Adult
2
Female
Adult
1
Male
Adult
2
Female
Adult
1
Unknown
Juvenile
3
Unknown
Unknown
2
Kuna Butte Study Area Pulex irritans
Aetheca wagneri
Grand View Study Area Pulex irritans
Meringis hubbardi
Strigiphilus speotyti
Family: Carnidae
66 identify these flies as carnids, which are hematophagous parasites of birds (Grimaldi 1997). Levels of Infestation Ectoparasite loads (levels 1, 2, or 3) per brood averaged 1.91 ± 0.15 (N = 33 broods) in 1997 and 1.87 ± 0.13 (N = 48 broods) in 1998. There was no significant interaction between year and chamber size (F2, 35 = 0.01, P = 0.99; Fig. 2.1) or between year and tunnel diameter (F1, 36 = 1.15, P = 0.29; Fig. 2.1) for brood ectoparasite loads. A significant interaction for either test would have indicated a differential effect of the treatment levels (year or ABS configuration) on ectoparasite loads of burrowing owl broods. Relative numbers of ectoparasites on a bird's head or on my hand also did not differ between tunnel diameters (Wilks' Lambda = 0.99, F2, 36 = 0.15, P = 0.86) or among chamber sizes (Wilks' Lambda = 0.87, F4, 58 = 1.01, P = 0.41; Fig. 2.2). Site Reuse Ectoparasite loads from 1997 did not seem to influence tunnel or chamber selection in 1998. There was no correlation between a nest's ectoparasite load in 1997 and its load in 1998 (rs = 0.19, P = 0.39, N = 22) for all reused nests considered together. In reused clusters of two ABSs, no significant correlation existed for ectoparasite loads between years (rs = -0.22, P = 0.55, N = 10). Seven owl pairs in 1998 used the same tunnel diameter as the 1997 pair. Six of these nests had low ectoparasite loads and one had a medium load in 1997. Three pairs in 1998 used tunnel diameters different from 1997 pairs, even though all three nests had low ectoparasite levels in 1997 (Table 2.2a). This distribution of tunnel-diameter reuse did not differ significantly from random (Mantel-Haenszel χ2 = 0.43, df = 1, P = 0.51). Similarly, no significant correlation
67 A.
3
Ectoparasite Level
1997 1998 2
1
8
0
18
4
10-cm
10 15-cm
Tunnel Diameter B.
Ectoparasite Level
3
1997 1998
2
1
2 0
Smal
5
3
2
Medium
16
13
Large
Chamber Size Figure 2.1. (A) Ectoparasite levels (mean ± SE) in 1997 and 1998 for burrowing owl broods in 19-L buckets with 10-cm and 15-cm diameter tunnels were not significantly different. (B) Ectoparasite levels (mean ± SE) in 1997 and 1998 for broods in small, medium, and large chambers were not significantly different. Sample sizes are indicated within bars.
68 A.
8
No. of Ectoparasites
7
Number on Head Number on Hand
6 5 4 3 2 1 25
25
14
14
0 10-cm
15-cm
Tunnel Diameter B.
14
No. of Ectoparasites
12
Number on Head Number on Hand
10 8 6 4 2 0
6
6 Smal
3
3 Medium
24
24
Large
Chamber Size Figure 2.2. (A) Number (mean ± SE) of ectoparasites counted from heads of juvenile burrowing owls and my hand for nests with different tunnel diameters. (B) Number (mean ± SE) of ectoparasites counted from heads of juvenile burrowing owls and my hand for nests with different chamber sizes. Sample sizes (broods) are indicated above or within bars.
69 Table 2.2a. Patterns of ectoparasite load and site reuse within clusters of two artificial burrow systems that burrowing owls used for nesting in 1997 and 1998. Nest Name
Ectoparasite Load 1997
Ectoparasite Load 1998
Same Tunnel Diameter Both Years?
EFN #2
Low
Low
Yes
EFS #2
Low
Low
Yes
Kuna Cave Satellite #1
Low
Medium
Yes
Kuna Butte Ag #1
Low
Low
No
Powerline #2
Medium
Low
Yes
Powerline #3
Low
Low
Yes
Powerline #5
Low
High
Yes
Baha Pole #19
Low
High
No
Well #3
Low
Low
No
Baha #3
Low
Low
Yes
70 Table 2.2b. Patterns of ectoparasite load and site reuse within clusters of three artificial burrow systems that burrowing owls used for nesting in 1997 and 1998. Nest Name
Ectoparasite Load 1997
Ectoparasite Load 1998
Same Chamber Size Both Years?
Medium
Low
No
Swan Falls #4
Low
High
Yes
Substation #2
High
High
Yes
Substation #5
Medium
Low
No
Substation #6
High
High
Yes
Grand View #19
High
High
Yes
Trailer View #1
High
Low
No
Baha #1
High
Medium
Yes
Substation East #1
Medium
Low
Yes
Trailer #1
Medium
High
Yes
Trailer #2a
Medium
Low
No
Trailer #4
Medium
Low
No
EFS #1
a
pair from Trailer #4 re-nested here after failing in their first attempt in 1998.
71 existed for ectoparasite loads between years (rs = -0.27, P = 0.39, N = 12) for reused clusters of three ABSs. Seven owl pairs in 1998 used the same chamber size as the 1997 pair. One nest had low, two had medium, and four nests had high ectoparasite loads in 1997. Five pairs in 1998 used chamber sizes different from 1997 pairs; four pairs had medium ectoparasite loads and one pair had a high ectoparasite load in 1997 (Table 2.2b). This distribution of chamber-size reuse did not differ significantly from random (MantelHaenszel χ2 = 2.31, df = 1, P = 0.13). Therefore, ABS reuse in 1998 was independent of ectoparasite load in 1997 for both cluster types. Juvenile Growth and Body Condition Ectoparasite load did not affect growth in juvenile owls (mass and tenth primary feathers) when analyzing brood averages (Wilks' Lambda = 0.94, F4, 134 = 1.37, P = 0.40) and using age and brood size as covariates (Fig. 2.3). Ectoparasite load also did not affect the number of fledglings per nest (F2, 78 = 1.48, P = 0.23). Overall, 5.39 ± 0.47, 5.61 ± 0.67, and 4.33 ± 0.54 juveniles fledged from nests with low, medium, and high ectoparasite levels, respectively. In my ectoparasite manipulation experiment, average age of owls from dusted nests was 17.3 d (range: 13 – 19 d) whereas juveniles from infested nests averaged 13.7 d (range: 10 – 16 d) on my first visit to the nest. I dusted three nests to eliminate ectoparasite loads (low, low, and high levels) and did not manipulate ectoparasite levels at two nests (medium and high). Owls or broods dusted with insecticide exhibited lower ectoparasite levels on subsequent captures. All three of my treatment broods exhibited lower ectoparasite loads on my second visit to the nest. One treatment nest decreased
72
140 Tenth Primary Mass
120
80
100 60
8 6
40
Mass (g)
Length of Tenth Primary (mm)
100
4 20
0
2 34
36 1
16
18 2
23
27
0
3
Ectoparasite Level
Figure 2.3. Mass (mean ± SE) and length (mean ± SE) of tenth primary of juvenile burrowing owls when analyzing brood averages. Means did not differ for either measure of growth (P > 0.05). Sample sizes are indicated within each bar.
73 from a high to low load and two nests decreased from low loads to zero ectoparasites detected. Hematocrit levels did not differ (F1, 3 = 0.45, P = 0.55) between dusted and infested nests when means for broods were analyzed (Fig. 2.4). Also, there was no significant interaction between treatment and time for hematocrit levels (F1, 3 = 0.25, P = 0.65). A significant interaction would have indicated a differential effect of the treatment level on hematocrit levels in juvenile owls. Finally, neither mass (F1, 3 = 0.48, P = 0.54) nor length of tenth primary (F1, 3 = 0.71, P = 0.46) differed between dusted and infested nests when means for broods were analyzed, nor were there any significant interactions between treatment and time (F1, 3 = 0.08, P = 0.79; F1, 3 = 0.30, P = 0.62, respectively; Fig. 2.5).
Discussion Ecology of Ectoparasites Collected Formerly in Idaho, only two species of flea had been collected from burrowing owls or their nests: Pulex irritans and Foxella ignota (Baird and Saunders 1992). These two species, and the two other species of flea I collected, typically are found on mammals that inhabit the drier regions of western North America. Pulex irritans is mainly a parasite of wild carnivores and especially is common on those that live in burrows or caves (R.E. Lewis, pers. comm.). In Idaho, this species has been found on American badgers, red foxes (Vulpes vulpes), coyotes (Canis latrans), deer mice (Peromyscus maniculatus), and burrowing owls. I found this species at very high densities in many burrowing owl nests in my study areas. On the other hand, neither Aetheca wagneri nor
74
0.7 0.6
Hematocrit (%)
0.5 0.4 0.3 0.2 0.1 2
3
Experimental Removal
Unaltered
0.0
Level of Treatment
Figure 2.4. Hematocrit levels (mean ± SE) of juvenile burrowing owls from nests where ectoparasites were experimentally removed and where natural levels remained unaltered. No significant differences were detected (P > 0.05). Samples sizes are indicated within each bar.
75
Length of Tenth Primary (mm)
A. 80
Dusted
60
Unaltered
40
20
0 1
2
Treatment Time B.
150 140 Dusted
Mass (g)
130 120 110
Unaltered
100 90 80 70 1
2
Treatment Time
Figure 2.5. (A) Length of tenth primary (mean ± SE) as a function of treatment time and level of treatment. (B) Mass (mean ± SE) as a function of treatment time and level of treatment. Average age of broods was used as a covariate in these analyses. Interactions were not significant for either measure of growth (P > 0.05).
76 Meringis hubbardi previously had been collected from burrowing owls in Idaho. Aetheca wagneri probably was an accidental associate of burrowing owls as it normally parasitizes small rodents such as deer mice, harvest mice (Reithrodontomys megalotis), and voles (Microtus sp.; Baird and Saunders 1992, R.E. Lewis, pers. comm.). Each of these small mammals is relatively abundant in my study areas. Meringis hubbardi probably was an accidental associate of burrowing owls as well (R.E. Lewis, pers. comm.). This species usually parasitizes kangaroo rats (Dipodomys sp.), but it also has been collected from deer mice, harvest mice, and Townsend's ground squirrels in Idaho (Baird and Saunders 1992). The importance of these species of flea as ectoparasites of burrowing owls is relatively uncertain. Because their normal hosts are mammals, the fleas may be unable to acquire food from burrowing owls. I observed only one flea that appeared to be feeding on a burrowing owl. This flea was found on the brood patch (a highly vascularized area) of a female owl during the incubation period, but I do not know if the flea was acquiring a blood-meal or simply happened to be on the brood patch when I captured the owl. Instead of a source of food, fleas may use the owls as phoretic hosts after their mammalian host has succumbed to predation. Fleas also may rely on the owls for transport out of the nest burrow so they can begin their search for a new host. However, even if these species of flea cannot acquire food from burrowing owls, they most likely benefit from the association in other ways. For example, I observed gravid females and mating pairs (probably P. irritans) on juvenile and adult burrowing owls. Owls likely provided thermoregulatory benefits and protection from predators to these fleas. Also, the nest substrate may provide adequate habitat for developing flea larva. Finally, two of
77 the three flea species I collected from burrowing owls (P. irritans and A. wagneri) and Foxella ignota are important in the transmission and maintenance of plague in nature (Baird and Saunders 1992). Therefore, understanding the role of burrowing owls in the life histories of these species also has important epidemiological ramifications. The lice specimens I collected were all Strigiphilus speotyti. This species is commonly found on burrowing owls and is highly specific to this host (Clayton 1990). I sporadically found this species on owls in my study areas. There were few (if any) individual lice per bird, and these often attached themselves to shafts of underwing coverts or primary feathers of adult owls. Only one owl I handled had an extremely high level of lice. In 1998, I captured an adult female burrowing owl that had recently completed her clutch of 11 eggs. Her right underwing harbored > 30 individual lice, and the feathers were in fair condition. Her left underwing harbored > 10 individual lice but showed extreme signs (denuded feather shafts, poor feather condition) of infestation. This nest failed soon after the eggs hatched, but it remains unclear if the high parasite load of the female affected the pair's reproductive attempt. I collected four specimens of S. speotyti from this owl: one adult female and three immature lice of unknown sex. The presence of immature lice indicates that successful reproduction of S. speotyti likely occurs on burrowing owls from southwestern Idaho. I collected carnid flies from only one owl, a juvenile whose feathers were not fully emerged. Carnus hemapterus, a carnid of eastern and northern North American, and C. occidentalis, of western North America, tend to occur on nestlings of cavitynesting birds or those species with nests protected from the elements (see Capelle and Whitworth 1973 in Dawson and Bortolotti 1997, Grimaldi 1997). An overlap occurs in
78 the distribution of these two species (Grimaldi 1997) so at this time, it remains unclear which species I collected from my study area. Carnid flies usually are found on nestlings prior to emergence of contour feathers, and their level of infestation steadily declines after pennaceous feather growth (Dawson and Bortolotti 1997). Carnid flies may have occurred on other nestling owls, but they were likely in low abundance as these specimens were the only ones noted after handling several hundred juvenile owls. Carnid flies usually occur at low densities (Dawson and Bortolotti 1997) and show no adverse effects on their hosts (see Kirkpatrick and Colvin 1989 in Dawson and Bortolotti 1997, Grimaldi 1997; but see Cannings 1986). Therefore, carnid flies as an ectoparasite of burrowing owls in southwestern Idaho are probably relatively unimportant. Levels of Infestation I detected no significant interaction between year and tunnel diameter or between year and chamber size for ectoparasite load of broods. Moreover, I detected no differences between tunnel diameters or among chamber sizes in the relative number of ectoparasites counted on a bird’s head or on my hand. Therefore, the two tunnel diameters and three chamber sizes I used for this experiment had no effect on the overall abundance of ectoparasites in nests occupied by burrowing owls. Because many ectoparasites overwinter in nests used during the breeding season (Moss and Camin 1970, Brown and Brown 1986, Barclay 1988, Møller 1989, Loye and Carroll 1991, Thompson and Neill 1991, Oppliger et al. 1994, Johnson 1996), I removed any nesting material remaining from 1997 prior to the 1998 breeding season. Thus, no differences in ectoparasite levels between years reflects either similar numbers of ectoparasites (i.e., fleas) each year, increased dispersal from or reduced immigration to
79 burrows by fleas (Heeb et al. 1996), or lower reliance on mammalian prey by adult owls in 1998. Recall that the three species of flea I collected are primarily associated with rodents. Burrowing owls commonly cache prey items inside the nest burrow (Haug et al. 1993, pers. obs.) for later consumption; most likely, fleas from mammalian prey items leave their deceased host and climb onto juvenile owls or remain on the nest substrate. In 1998, I observed fewer mammals cached within artificial burrow chambers and typically fewer juveniles per nest than in 1997. Therefore, fewer sources of fleas (i.e., cached prey) in 1998 may have led to the similarities I observed in ectoparasite loads. Also, had I not cleaned out nesting material from 1997, ectoparasite levels in 1998 might have increased because of overwinter survival of adults (which can survive extended periods without feeding), eggs, and larvae (Butler and Roper 1996, Heeb et al. 1996, Rendell and Verbeek 1996). Removal of nesting material likely influences flea numbers in burrow systems (Butler and Roper 1996), but other ectoparasites (i.e., lice) likely infest nest sites randomly because their overwinter survival is independent of old nest material (Rendell and Verbeek 1996). Site Reuse Burrow reuse in 1998 was not influenced by ectoparasite load in 1997. Burrowing owls commonly reuse nest burrows from year to year in and around my study areas (Rich 1984, King 1996, Belthoff and King 1997, Belthoff and Smith 1998, Belthoff and Smith 1999). Reuse of a nest site may be predicated by previous success at the site or changes in quality at the site (Gowaty and Plissner 1997). For example, Loye and Carroll (1991) found that cliff swallows (Hirundo pyrrhonata) used bridge colonies every year, whereas cliff colonies were not used for at least one year after occupation. Bridge
80 colonies supported significantly lower populations of cliff swallow bugs (Oeciacus vicarius) than cliff colonies just prior to colony selection by swallows (Loye and Carroll 1991). Swallow bug populations diminished in cliff colonies with discontinuous occupation, which suggested that swallows selected colonies that had been unoccupied for at least one season to reduce any potential effects of ectoparasites (Loye and Carroll 1991). Conversely, some bird species prefer sites that were used during the previous breeding season. Davis et al. (1994) found that eastern bluebirds (Sialia sialis) preferred boxes containing old nests and hypothesized that adults use old nests to benefit from wasps (Nasonia vitripennis) that kill the pupae of a significant nestling parasite, the blowfly (Protocalliphora sialis). Similarly, house wrens (Troglodytes aedon) do not prefer boxes cleaned of any nesting material more than boxes with previous nesting material intact (Thompson and Neill 1991, Johnson 1996). Thompson and Neill (1991) suggest that wrens measure a site's suitability based on the amount of fecal material from the previous season; pairs then remove some of the old material and reconstruct the nest cup with new material. Johnson (1996) also noted that wrens clearly avoided boxes with old nests containing large quantities of fecal material and speculated that too much fecal material made removal of nest material unusually difficult. Because ectoparasite load did not influence nest-site selection in my study and burrowing owls commonly reuse natural burrows, ectoparasite levels within a nest burrow during nest-site selection may not be as important as other burrow characteristics (see Chapter One) or habitat preferences. Juvenile Growth and Body Condition Ectoparasite load did not have significant effects on mass of juvenile owls, length of tenth primary feathers, or number of juveniles fledged per nest, although broods with
81 high loads of ectoparasites had shorter tenth primary feathers and fledged fewer young per nest. In contrast, nestlings of purple martins (Progne subis), barn swallows (Hirundo rustica), prairie falcons (Falco mexicanus), blue tits, and cliff swallows infested with ectoparasites weigh significantly less than uninfested young (Moss and Camin 1970, Brown and Brown 1986, Møller 1987, Chapman and George 1991, Dufva and Allander 1996, McFadzen and Marzluff 1996, but see Tripet and Richner 1997). In addition to reduced mass and hematocrit levels of infested prairie falcon nestlings, some juveniles were found dead after they had jumped from heavily infested nest-sites to escape high levels of ectoparasitism (McFadzen and Marzluff 1996). Also, cliff swallow nestlings with high levels of ectoparasitism had shorter primaries than those with experimentally reduced levels of ectoparasites (Chapman and George 1991). Therefore, it appears that ectoparasite load did not influence growth and survival of juvenile burrowing owls as it has been found to in other species of birds. Finally, it is unclear if the fleas I observed on burrowing owls actually acquire food from them. Evidence from the hematocrit levels in my infested and dusted groups suggests that the fleas may not be acquiring significant blood-meals as I detected no difference in the packed-cell volume between the two groups. However, my sample size for this experiment was small (N = 5 nests), and I may have failed to detect a difference (Power = 0.13). Nonetheless, ectoparasites likely have negative effects on juvenile burrowing owls. Highly infested owls may preen more often (Clayton 1991), exhibit reduced amounts of sleep (Christe et al. 1996b), or expend more energy by increasing their frequency of begging vocalizations (Christe et al. 1996a). These potential forms of energy loss, which my study did not measure, could lead to an energy deficit in highly
82 ectoparasitized birds, causing slower growth rates, extended nestling stages, or increased mortality (Chapman and George 1991).
Conclusions My study identified five species of ectoparasites from burrowing owls in southwestern Idaho. I collected the flea P. irritans most often, and S. speotyti was the only species of louse found in either study area. I also collected a species of parasitic fly (Family: Carnidae) from a juvenile owl in Grand View. The fleas I collected normally are parasites of small mammals common in and around the study areas and probably infest burrowing owls after their normal hosts (mammals) become prey items for owls or when these small mammals use burrows as shelter. It remains uncertain if these fleas acquire nutrition from burrowing owls, but they may benefit from the association through protection from predators, phoresis, or by laying eggs in nest material. The species of louse I collected is highly specific to burrowing owls and occurred sporadically on owls in my study areas. Ectoparasite loads at ABSs did not influence site reuse between years. Ectoparasite loads also did not affect juvenile growth (mass and length of tenth primary) or number of fledglings per nest. Although I did not detect a difference in hematocrit levels between owls with and without ectoparasites, it is likely that very high levels of ectoparasites negatively affect body condition in burrowing owls (e.g., the adult female infested with lice almost certainly suffered body condition effects of high louse load). High levels of ectoparasitism negatively affect rates of growth and survival, nestling time budgets, nest-site reuse, and dispersal patterns in several other bird species, and likely
83 influence burrowing owls in similar fashions. To thoroughly examine effects of ectoparasites on burrowing owls, ectoparasite levels could be manipulated to increase loads for some broods, while eliminating ectoparasites in others. However, because of the sensitive nature of the study species, this type of manipulation was impractical, which limited the experimental approaches I could use in this experiment. Based on my studies, it appears that burrowing owls in southwestern Idaho are exposed to various levels of flea infestation throughout their life. Although I did not detect negative effects of ectoparasites on growth or health in juvenile owls, the ultimate effects (survival and fecundity) of ectoparasitism remain unknown.
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86 Kirkpatrick, C.E., and B.A. Colvin. 1989. Ectoparasitic fly Carnus hemapterus (Diptera: Carnidae) in a nesting population of common barn-owls (Strigiformes: Tytonidae). J. Med. Entomol. 26:109-112. Loye, J.E., and S.P. Carroll. 1991. Nest ectoparasite abundance and cliff swallow colony site selection, nestling development, and departure time. Pp. 222-241. In: J.E. Loye and M. Zuk, eds. Bird-parasite interactions: ecology, evolution and behavior. Oxford Univ. Press, New York. Mayer, W.V. 1955. The protective value of the burrow of the hibernating arctic ground squirrel, Spermophilus tridecemlineatus. Anat. Rec. 122:437-438. McFadzen, M.E., and J.M. Marzluff. 1996. Mortality of prairie falcons during the fledging-dependence period. Condor 98:791-800. Møller, A.P. 1987. Advantages and disadvantages of coloniality in the swallow, Hirundo rustica. Anim. Behav. 35:819-832. Møller, A.P. 1989. Parasites, predators, and nest boxes: facts and artifacts in nest box studies of birds. Oikos 56:421-423. Møller, A.P. 1990. Effects of parasitism by the haematophagous mite on reproduction in the barn swallow. Ecology 71:2345-2357. Møller, A.P. 1993. Ectoparasites increase the cost of reproduction in their hosts. J. Anim. Ecol. 62:309-322. Møller, A.P., F. de Lope, J. Moreno, G. Gonzalez, and J.J. Perez. 1994. Ectoparasites and host energetics: house martin bugs and house martin nestlings. Oecologia 98:263-268. Moss, W.W., and J.H. Camin. 1970. Nest parasitism, productivity, and clutch size in purple martins. Science 168:1000-1003. Nilsson, S.G., K. Johnson, and M. Tjernberg. 1991. Is avoidance by black woodpeckers of old nest holes due to predators? Anim. Behav. 41:439-441. NOAA (National Oceanic and Atmospheric Administration). 1985. Climates of the States: 1951-1980. Vol. 1, 3rd ed. 758 pp. Nur, N. 1988. The cost of reproduction in birds: an examination of the evidence. Ardea 76:155-168.
87 Oppliger, A., H. Richner, and P. Christe. 1994. Effect of an ectoparasite on lay date, nest site choice, desertion, and hatching success in the great tit, Parus major. Behav. Ecol. 5:130-134. Oppliger, A., P. Christe, and H. Richner. 1997. Clutch size and malarial parasites in female great tits. Behav. Ecol. 8:148-152. Philips, J.R., and D.L. Dindal. 1977. Raptor nests as a habitat for invertebrates: a review. Raptor Res. 11:87-96. Price, P.W. 1980. Evolutionary biology of parasites. Princeton Univ. Press, Princeton, New Jersey. Rendell, W.B., and N.A.M. Verbeek. 1996. Are avian ectoparasites more numerous in nest boxes with old nest material? Can. J. Zool. 74:1819-1825. Rich, T. 1984. Monitoring burrowing owl populations: implications of burrow re-use. Wildl. Soc. Bull. 12:178-180. Richner, H., A. Oppliger, and P. Christe. 1993. Effect of an ectoparasite on reproduction in great tits. J. Anim. Ecol. 62:703-710. Roby, D.D., K.L. Brink, and K. Wittmann. 1992. Effects of bird blowfly parasitism on eastern bluebird and tree swallow nestlings. Wilson Bull. 104:630-643. Sonerud, G.A. 1993. Reduced predation by nest box relocation: differential effect on Tengmalm's owl nests and artificial nests. Ornis Scand. 24:249-253. Thomsen, L. 1971. Behavior and ecology of burrowing owls on the Oakland municipal airport. Condor 73:177-192. Thompson, C.F., and A.J. Neill. 1991. House wrens do not prefer clean nestboxes. Anim. Behav. 42:1022-1024. Tripet, F., and H. Richner. 1997. Host responses to ectoparasites: food compensation by parent blue tits. Oikos 78:557-561. Turner, Jr., E.C. 1971. Fleas and lice. Pp. 175-184. In: J.W. Davis, R.C. Anderson, L. Karstad, and D. O. Trainer, eds. Infectious and Parasitic Diseases of Wild Birds. Iowa St. Univ. Press, Ames, Iowa. Young, B.E. 1993. Effects of the parasitic botfly Philornis carinatus on nestling house wrens, Troglodytes aedon, in Costa Rica. Oecologia 93:256-262.
88 Zar, J.H. 1996. Biostatistical analysis. 3rd edition. Prentice-Hall, Inc., Upper Saddle River, New Jersey. Zarn, M. 1974. Burrowing owl, Athene cunicularia hypugaea. Report No. 11, Habitat management series for unique or endangered species. Bureau of Land Mgmt., Denver, Colorado. 25 pp.
89
CHAPTER 3: BURROWING OWLS AND HUMAN DEVELOPMENT: RESULTS OF SHORT-DISTANCE NEST BURROW RELOCATIONS TO AVOID CONSTRUCTION IMPACTS
Abstract Five burrowing owl (Athene cunicularia) nests in artificially constructed burrows faced destruction by development in southwestern Idaho during June - July 1998. Regulatory agencies agreed that active relocation of the nest burrows would simultaneously allow construction to proceed and provide an opportunity to determine the efficacy of moving occupied burrowing owl nests as a mitigation alternative. Relocated nests contained one to five nestlings, which ranged in age from 27 – 45 d. At these ages, young are capable of modest to good flight, depend on parental care to varying extents, and remain associated with natal burrows. Nest burrows (plastic chamber and tunnel) and wooden perches were relocated to adjacent buffer strips that contained natural vegetation and, to reduce the likelihood that owls would return to original nest locations, access to natural burrows near original nest locations was restricted to the extent possible. Relocation distances averaged 153 m and ranged from 72 – 258 m. Because terrain was flat (disked field dominated by non-native grasses and forbs), new nest locations generally were within view of the original location. I did not
90 move adult owls but expected them to travel the fairly short distances to new burrow locations on their own. Relocations were successful (i.e., family remained at the new location) at two of the five nests. For two other nests, both adults and young returned to the vicinity of the original nest and occupied natural burrows the day following relocation. I did not detect owls from the fifth nest in the days and weeks following burrow relocation and presumed they dispersed from the immediate vicinity of the construction.
Introduction Burrowing owl (Athene cunicularia) populations are declining throughout much of their North American range (De Smet 1997, James and Espie 1997, Sheffield 1997). Human disturbances, such as elimination of burrowing mammals, pesticide and herbicide use, and habitat loss through grassland conversion to agriculture or urbanization, are among the factors contributing to the declines (Zarn 1974, Haug et al. 1993). Humaninfluenced habitat change is continually displacing owls, forcing them from the previous season’s nesting areas, reducing prey abundance and foraging areas, and potentially limiting opportunities for breeding. Although federal regulations protect the owls, situations continually arise where burrowing owls and land uses conflict. To minimize direct impacts to burrowing owls resulting from habitat conversion to agriculture or development, mitigation efforts often attempt to provide burrowing owls with suitable habitat outside of a development area. Once mitigation land is acquired and established near an impact area, owls are either passively or actively relocated (Trulio 1995, Feeney 1997). Passive relocation usually occurs in the non-breeding season or just
91 before the breeding season commences. Under this scenario, owls are excluded from available natural burrows in development areas and are passively forced to seek burrows in nearby habitat outside of areas directly affected by construction. Active relocation entails (1) capturing owls and moving them to suitable habitat well removed from the original site, and (2) releasing the owls at the new site, often after a period of acclimation in temporary aviaries. To replenish or re-establish populations, burrowing owls also are translocated into areas where suitable habitat remains but natural populations decline or are extirpated (Martell 1990, Dyer 1991). Translocation projects require capture and transport of adults and juveniles from breeding areas and release in establishment sites. The efficacy of these mitigation techniques (active relocation, passive relocation and translocation) has varied. Many relocation projects result in fewer breeding pairs of burrowing owls at the mitigation site than at the original site, and translocation projects generally fail to produce self-sustaining populations. Investigators attribute the limited success of these management efforts to (1) strong site tenacity by adult owls, and (2) potential risks associated with forcing owls to move into unfamiliar and perhaps less preferable habitats (Trulio 1995, Delevoryas 1997, Feeney 1997). Developing methods to increase the efficacy of burrowing owl relocation and translocation needs to be a priority but requires research studies incorporating these methods. In this study, I designed an experiment to examine how burrowing owl families respond to short-distance nest burrow relocation. I compared the distance each burrow was moved, the number of juveniles in each nest, and the characteristics describing successful and unsuccessful relocations. I also tested the hypothesis that adult burrowing owls would move short distances to new nest sites, perhaps because they were enticed by
92 the vocalizations of their young, which would eliminate the need to capture each individual adult. I conducted this experiment in response to the imminent destruction of a 130-ha field in which five pairs of burrowing owls nested in 1998. Each of the five nests contained a pair of adults and a number of dependent fledglings still closely associated with the nest burrow. Before young were ready to leave their natal area, the field became a borrow pit for construction of a wastewater treatment facility; ultimately, the field will function as an effluent field in which alfalfa and other cover crops are grown. To allow the project to proceed while minimizing impacts to the nests, state and federal regulatory agencies agreed that the situation offered an opportunity to examine the feasibility of active relocation of burrowing owl nests to avoid construction impacts. All involved concurred that the nests would be relocated to the periphery of the construction project, into a buffer strip surrounding the field. Such nest relocations would allow construction to continue without costly delays that would result from waiting until the owls migrated from the construction area in fall. Therefore, this study summarizes the relocation of occupied nests in an attempt to determine if active relocation of burrowing owls nests is a feasible mitigation alternative that could avoid or reduce direct impacts from construction.
Study Area and Methods Study Area The five burrowing owl nests were located approximately 3.2 km south of Kuna (43° 25' N, 116° 25' W), Ada County, which is 32 km southwest of Boise, Idaho and 23
93 km north of the Snake River Canyon. The topography of the area is flat to rolling with elevations ranging from 841 m to 896 m. Rock outcrops and a few isolated buttes (e.g., Kuna Butte, elev. 986 m) exist in the region. Temperatures range from -20° to 45° C, and annual precipitation typically averages less than 20 cm (NOAA 1985). The study area was once a typical shrubsteppe community dominated by large expanses of big sagebrush (Artemisia tridentata wyomingensis; Hironaka et al. 1983). Range fires and other disturbances have converted much of the surrounding shrublands to exotic grasslands dominated by cheatgrass (Bromus tectorum) and tumble mustard (Sisymbrium altissimum). The area contained a few homes, several large dairy farms, paved and gravel roads, and irrigated agricultural fields that primarily grew alfalfa, mint, and sugar beets. Irrigated agricultural fields bordered three sides (east, south, and north) of the field under construction, and a well-traveled, two-lane road bordered its western edge. Previously excavated American badger (Taxidea taxus) burrows were high in density and served as the primary natural nest and shelter sites for burrowing owls (King 1996, Belthoff and King 1997). Nestling Data Prior to moving young from each nest burrow, I estimated the age of juveniles based upon feather growth (Landry 1979), relative size, two unpublished photographic keys, and estimated hatching dates obtained for a different study. Each owl received one United States Geological Survey aluminum leg band (size 4) and a unique color combination of three plastic leg bands (red, blue, green, yellow, pink, and white; National Band and Tag Co., Newport, KY) for visual identification in the field.
94 Nest Relocation The five nests occurred in artificial burrows that I deployed as part of another study (see Chapter One for details). Therefore, active relocation of nests and juveniles was relatively simple when compared with moving nests from natural burrows. Because I knew from experience that adults are difficult to capture once juveniles are about to fledge, and this project occurred approximately during that period of the nesting cycle, I did not capture adult owls, but expected them to move the short distances from original nest areas to relocation sites. Thus, my methods combined aspects of both active and passive relocation. They reflected active relocation in that nest burrows and juveniles were physically moved, but passive relocation methodology in that adult owls were not moved. All five nests were relocated to buffer strips surrounding the construction site (Fig. 3.1). I selected new nest locations that were as close as possible to the original nest location in areas that provided sufficient space and habitat. New sites generally were no closer to neighboring nests than they were originally (except for EFS #2 and EFS #3) and, in each case, the new nest locations were within view of the original sites. After site selection, I (1) dug holes into which I would replace artificial burrows, (2) removed all juveniles from their nest chamber, (3) removed nest burrows intact (i.e., the chamber and tunnel), (4) transported the artificial burrow materials to their new locations, (5) buried them at the new location using the same entrance orientation as the original burrow, and (6) replaced each juvenile in its nest chamber. I also moved the wooden perch from the original site to the new site to lure adult owls, who regularly used the perch for roosting, and marked each site with high visibility flagging to reduce chances that construction
95
Kuna
N Agriculture Field
EFN #2 EFN #3
Agriculture Field
EFS #1
Shrubsteppe Habitat
EFS #3
EFS #2
Agriculture Field
Original Location New Location Buffer Strip Improved Road Unimproved Road Fence
Shrubsteppe Habitat
Figure 3.1. Schematic of study area located approximately 3.2 km south of Kuna, Ada County, Idaho. Arrows indicate direction and location of relocated artificial nest burrows. See Table 3.1 for relocation distances and outcomes.
96 personnel would inadvertently disturb new sites. To determine the fate of each relocated nest, I monitored relocation areas each day after its relocation for two weeks, and at least three times per week thereafter. There is a tendency for fairly strong site attachment by burrowing owls (Trulio 1995, Delevoryas 1997, Feeney 1997), so it was possible that some owls would return to the location of their original nest after the nest burrow was removed. To minimize this possibility, first I placed Owl Exclusionary Devices (OED) at natural burrows near the original nest site. Each OED consisted of a 0.5-m section of perforated plastic drainage pipe and a piece of transparent Plexiglas® attached to a hinge at one end of the pipe. Once placed at the entrance to a natural burrow, OEDs allowed any owls that were underground to exit, but they prevented owls from taking up residence at such burrows. I also attempted to coordinate relocations such that original nest areas would be destroyed shortly after nest burrows were moved, and thus reduce the likelihood that owls would return to their original nest. Upon relocating each nest, I measured the distance (to nearest 0.5 m) and azimuth direction from the original nest location to its new site. I considered a relocation successful if an owl family took up residence at its respective new location. Unsuccessful relocations were those in which owl families returned to their original nest areas or immediately dispersed from the study area because dispersal at this age is not characteristic for this species (see King 1996).
97 Results Nestling Data The number of juveniles at each nest varied from one to five and ranged in age from 27 – 45 d (Table 3.1). At these ages, young are capable of modest to good flight, but they still depend on parental care and remain associated with natal burrows. I captured and relocated all juveniles with each nest except at Effluent Field South (EFS) #1 where, upon my approach to the nest, one young flew approximately 25 m away. At Effluent Field North (EFN) #2, both young were captured and relocated, but one juvenile flew across Swan Falls Road (which was in the opposite direction of the original site) immediately after I completed the relocation. Nest Relocation Relocation distances averaged 153 m, ranging from 72 m for EFS #1 to 258 m for EFN #3, and all but one nest were moved in a westerly direction (Table 3.1; Fig. 3.1). Only two of the five (40%) original nest areas were immediately destroyed after their relocations (EFN #2 and EFN #3), but all five original nest burrows had at least one natural burrow within 50 m. Overall, two families (40%) accepted the relocation site, two families (40%) returned to the area of their original nest burrow, and one family (20%) dispersed from the immediate vicinity (Table 3.1). There was no relationship between number of juveniles (F1, 3 = 0.27, P = 0.638), age of juveniles (F1, 3 = 2.70, P = 0.199), or distance of nest relocation (F1, 3 = 0.42, P = 0.562) and whether a relocation site was accepted. All family members from EFN #2 and EFS #1 occupied the new sites on the day following relocation, and both juveniles and adults from each family used the new site for several weeks until they dispersed. In contrast, two families returned to their
98
Table 3.1. Nestling information, relocation measurements, and fate of each relocated burrowing owl nest. Juveniles and artificial nest burrows were relocated during the 1998 breeding season to avoid construction impacts in Ada County, Idaho. Number of Nestlings
Agea (d)
Relocation Date
Distance (m)
Azimuth (°)
Fate
Effluent Field South #1
3
44 - 45
7 July
72.5
168
Accepted new site
Effluent Field South #2
5
35 - 38
7 July
79
251
Dispersed
Effluent Field South #3
1
27
9 July
183
242
Site tenacity
Effluent Field North #2
2
39 - 40
25 June
174
220
Accepted new site
Effluent Field North #3
4
38 - 39
25 June
258
218
Site tenacity
Nest Site Name
a
age estimated based on initiation of egg hatching and morphological development.
99 original nest locations and failed to reside in the relocation areas. On the day following relocation, family groups from EFN #3 and EFS #3 were at natural burrows near their respective original nest areas. However, the adult male from EFS #3 began using the perch, and possibly the artificial burrow, at the new site approximately 10 days after relocation, but his young and the adult female remained near the original nest. I believe the final family group, EFS #2, dispersed from the immediate vicinity of both the original nest and the relocated burrow, even though this nest was moved only 79 m from the original site. After moving the artificial burrow and all five juveniles, no members of the family occupied the original or relocation sites, nor were they in immediately surrounding areas with suitable habitat for burrowing owls.
Discussion Adult burrowing owls typically remain within 50 – 100 m of their nest or satellite burrows during daylight hours (Haug and Oliphant 1990) and tend to exhibit strong site tenacity to nest burrows, even after a site has been disturbed (Zarn 1974, Feeney 1997). Because burrowing owls commonly use burrows nearby their nest burrows for roosting, escape cover, and other activities (Zarn 1974, Haug et al. 1993, King 1996, Belthoff and King 1997), relocated nests likely should be in close proximity to the original nest burrow (Trulio 1995). For the two successful relocations in my study, burrows averaged 123 m from their original sites. For unsuccessful relocations, the distances averaged 173 m from their original sites. Both of these averages are greater than the 100-m maximum distance that Trulio (1995, 1997) recommends for passive relocation but, because shorter average movements were generally more successful, distance also might
100 be of concern for the type of relocations I employed (but see EFS #2 where family members that were moved only a short distance apparently dispersed from the study area). Burrowing owls commonly return to the same or nearby nest burrow year after year (Thomsen 1971, Rich 1984, King 1996, Belthoff and King 1997, Botelho and Arrowood 1998, Belthoff and Smith 1998, 1999). For the successful relocations, both adult males and one adult female bred successfully in this field during the previous (1997) breeding season (based on banding information). Such experience could have made these owls more familiar with relocation areas and led to their increased willingness to accept new, nearby sites. For the three unsuccessful relocations, only one adult male was known to have bred in this field during 1997, and his family dispersed from the field immediately following relocation. Therefore, familiarity with this field may have influenced whether a family accepted their relocation site, returned to the original nest area (i.e., site tenacity), or dispersed from the area although this inference is weak given the small sample size. Other factors, such as increased disturbance, also may have affected willingness to accept new sites (Feeney 1997). For example, because of the configuration of the buffer zones around the construction, two of the three unsuccessful relocations were moved from the eastern-most and “quietest” portion of the field to within 20 m of the busy road. Unfamiliar disturbances (e.g., traffic) near the new site could have caused the owls to reject the new sites. Finally, for the two successful relocations, one juvenile from each nest either was not captured or escaped during the relocation process. It is not clear if or why this would affect the tendency for families to remain in the relocation area. One hypothesis is that separation of family members led to
101 increased rate of contact vocalizations by juveniles, which lured adults to the new site more readily than in families where young remained together during the entire relocation process. One problem I encountered for both successful and unsuccessful relocations was the inability to have the original areas destroyed immediately. Because of inclement weather, construction was delayed (for a few days in some cases) and destruction of the field did not occur on planned dates. Delays in construction potentially influenced two families to either return to natural burrows near their original nest area or disperse from the immediate area (see EFS #3 and EFS #2, respectively). The family from EFS #3 moved to a natural burrow approximately 50 m from their original nest burrow. I had not placed an OED at this particular burrow because of its distance from the original nest burrow. The third family that did not accept the relocation site (EFN #3) returned to their original nest area despite removal of their nest burrow and destruction of the immediate vicinity. Unfortunately, the family returned to use a natural burrow that persisted after grading of the area had occurred. The burrow was approximately 10 m from their original nest burrow and its entrance was buried only a few centimeters by the construction equipment. The owls (or a fossorial mammal) re-excavated the entrance the day following the grading event and the family occupied the burrow. To increase the likelihood that new sites will be accepted, original nest areas ideally should be graded or otherwise thoroughly destroyed immediately after moving the owls so they cannot return to the original burrow, or any other burrow, in the impacted area (Trulio 1995). My feeling is that both of the families that returned to their original nest areas would have taken up residence at new sites were the areas destroyed
102 thoroughly in the planned fashion. It is unknown how the family that dispersed from the immediate area would have responded to thorough and immediate destruction of their original nest area. To conclude, my results indicate that short-distance relocation of occupied nests is successful under some circumstances. The relocations I performed avoided the almost certain death of many young owls that would have resulted from construction. However, this was a small study, restricted to only five nests. Ideally, success rates for the techniques described here need to be quantified from much larger studies before such relocations can be performed where impacts cannot otherwise be avoided. Also, it remains completely unknown whether the techniques I examined relate to owls nesting in natural burrows, even though this will be the most likely situation faced by resource managers in many areas. Finally, effects of moving adults along with young on the success of short distance relocations remain unknown. If the stress of capture did not frighten the birds too severely, it seems reasonable that including adults would increase relocation success. However, as in my study, it may be difficult to capture adults late in the nesting cycle, so timing of the relocation would be important. Hopefully, further research along the lines discussed here can help determine the efficacy of techniques that will reduce negative direct and indirect effects of land use change on burrowing owls.
Literature Cited Belthoff, J.R., and R.A. King. 1997. Between-year movements and nest burrow use by burrowing owls in southwestern Idaho: annual report for 1996. Unpubl. Tech. Rept. to Idaho Bureau of Land Mgmt. 28 pp.
103 Belthoff, J.R., and B.W. Smith. 1998. Monitoring between-year movements and assessment of artificial burrow features useful in conservation and management of burrowing owls. Unpubl. Tech. Rept. to Idaho Bureau of Land Mgmt. 31 pp. Belthoff, J.R., and B.W. Smith. 1999. Monitoring burrowing owls and assessment of artificial burrow features useful in conservation and management. Final Report. Unpubl. Tech. Rept. to Idaho Bureau of Land Mgmt. 54 pp. Botelho, E.S., and P.C. Arrowood. 1998. The effect of burrow site use on the reproductive success of a partially migratory population of western burrowing owls (Speotyto cunicularia hypugaea). J. Raptor Res. 32:233-240. De Smet, K.D. 1997. Burrowing owl (Speotyto cunicularia) monitoring and management activities in Manitoba, 1987 – 1996. Pp. 123-130. In: J.R. Duncan, D.H. Johnson and T.H. Nicholls, eds. Biology and conservation of owls of the northern hemisphere: proceedings of the second international owl symposium. U.S. Dept. of Agric. Gen. Tech. Rept. NC-190. Delevoryas, P. 1997. Relocation of burrowing owls during courtship period. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:138-144. Dyer, O. 1991. Reintroduction of burrowing owls (Athene cunicularia) to the South Okanagan Valley, British Columbia (1983-1988). Provincial Mus. of Alberta Nat. Hist. Occasional Paper No. 15. Feeney, L.R. 1997. Burrowing owl site tenacity associated with relocation efforts. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:132-137. Haug, E.A., and L.W. Oliphant. 1990. Movements, activity patterns, and habitat use of burrowing owls in Saskatchewan. J. Wildl. Manage. 54:27-35. Haug, E.A., B.A. Millsap, and M.S. Martell. 1993. Burrowing owl (Speotyto cunicularia). In: A. Poole and F. Gill, eds. The Birds of North America, No. 61. The Academy of Natural Sciences, Philadelphia; The American Ornithologists' Union, Washington, D.C. Hironaka, M., M.A. Fosberg, and A.H. Winward. 1983. Sagebrush-grass habitat types of southern Idaho. Forest, Wildl. and Range Expt. Stat., Univ. of Idaho, Moscow, Bull. No. 35. 44 pp. James, P.C., and R.H.M. Espie. 1997. Current status of the burrowing owl in North America: an agency survey. In: J.L. Lincer and K. Steenhof, eds. Proceedings of the first international burrowing owl symposium. J. Raptor Res. Report 9:3-5.
104 King, R.A. 1996. Post-fledging dispersal and behavioral ecology of burrowing owls in southwestern Idaho. M.S. Thesis, Boise State Univ., Boise, Idaho. Landry, R.E. 1979. Growth and development of the burrowing owl. M.S. Thesis, California State Univ., Long Beach. Martell, M.S. 1990. Reintroduction of burrowing owls into Minnesota: a feasibility study. M.S. Thesis, Univ. of Minnesota, Minneapolis. NOAA (National Oceanic and Atmospheric Administration). 1985. Climates of the States: 1951-1980. Vol. 1. 3rd ed. 758 pp. Rich, T. 1984. Monitoring burrowing owl populations: implications of burrow re-use. Wildl. Soc. Bull. 12:178-180. Sheffield, S.R. 1997. Current status, distribution, and conservation of the burrowing owl (Speotyto cunicularia) in midwestern and western North America. Pp. 399-407. In: J.R. Duncan, D.H. Johnson and T.H. Nicholls, eds. Biology and conservation of owls of the northern hemisphere: proceedings of the second international owl symposium. U.S. Dept. of Agric. Gen. Tech. Rept. NC-190. Thomsen, L. 1971. Behavior and ecology of burrowing owls on the Oakland municipal airport. Condor 73:177-192. Trulio, L.A. 1995. Passive relocation: A method to preserve burrowing owls on disturbed sites. J. Field Ornithol. 66:99-106. Trulio, L.A. 1997. Strategies for protecting western burrowing owls (Speotyto cunicularia hypugaea) from human activities. Pp. 461-465. In: J.R. Duncan, D.H. Johnson and T.H. Nicholls, eds. Biology and conservation of owls of the northern hemisphere: proceedings of the second international owl symposium. U.S. Dept. of Agric. Gen. Tech. Rept. NC-190. Zarn, M. 1974. Burrowing owl, Athene cunicularia hypugaea. Report No. 11, Habitat management series for unique or endangered species. Bureau of Land Mgmt., Denver, Colorado. 25 pp.
105
GENERAL CONCLUSIONS Despite the ecological importance of nest-site selection for a species, few experimental studies have provided organisms a range of characteristics from which to choose their nest-site. This type of approach allows the organism to select the characteristic(s) they prefer from choices available to them, thus allowing researchers to determine nest-site characteristics preferred by the species. A basic understanding of an organism's nest-site preferences provides information useful to the conservation and management of that species and its habitat. This basic understanding is critical in the management of burrowing owl populations, especially in those regions where their populations have declined. In my study areas, the population appears to be relatively stable, but little or no data from long-term monitoring programs are available for comparison. For secondary cavity-nesting species, space within the cavity for adults and juveniles may be critical to their nest success, survival, and energetic demands. However, there likely exists a threshold of cavity size (too large or too small) where the organism loses any benefits derived from an optimum chamber size. These losses may include reduced levels of fitness and survival, increased levels of ectoparasitism, or thermoregulatory constraints. In the experiment described in Chapter One, nesting pairs of burrowing owls nested in large chambers and small-diameter tunnels more often than other configurations available to them. From the experimental results and my
106 field observations I conclude that (1) large chambers reduce negative effects of overcrowding within the nest burrow, (2) small-diameter tunnels may exclude large mammalian predators from entering an ABS, (3) because burrowing owls commonly nest in previously occupied burrows, such sites need to be protected from destruction, and (4) managers should consider deploying ABSs with large chambers and small-diameter tunnels to increase the likelihood of ABS occupancy. Space available within the burrow system likely does not influence nest-site selection on its own. Other variables, such as ecological, social, behavioral, or physiological factors, likely affect decisions concerning which burrow owls choose as nest-sites. For example, not all breeding pairs selected large chambers as characteristics they preferred. Perhaps these owls preferred the microclimate offered by the other configurations or felt that space was not a constraint within the chamber. Also during my study, nesting pairs did not occupy many previously used sites despite offering chamber sizes that were used by dozens of owl pairs. Perhaps the habitat surrounding these sites had changed, thus influencing prey populations. I was unable to test for differences in ABS microclimate or in above-ground habitat quality (prey abundance and availability, cover, etc.) around each previously occupied nest, so hypotheses concerning these factors remain tenable. The effects of ectoparasites on avian hosts are not well understood but may be critical to their survival and reproductive success, and likely influence certain behavior patterns. In Chapter Two I describe the ecology of several species of ectoparasites I collected from burrowing owls in southwestern Idaho and discuss their abundance and effects on broods of burrowing owls. Ectoparasites of burrowing owls (1) included three
107 species of flea, one species of louse, and one species of carnid fly, (2) have various normal hosts (the fleas originate from mammalian hosts whereas the louse is highly specific to burrowing owls), (3) occurred at various densities in each type of ABS, and (4) did not affect nest-site reuse, nestling growth or hematocrit levels, or the number of fledglings per nest. Failure to detect any differences in growth among various ectoparasite levels may have resulted from a lack of statistical power or a true lack of effect on juvenile growth. Further investigations should help clarify the ecological and physiological role(s) of ectoparasites on burrowing owls. High levels of ectoparasitism negatively affect rates of growth and survival, nestling time budgets, nest-site reuse, and dispersal patterns in several other bird species, and likely influence burrowing owls in similar fashions. To thoroughly examine effects of ectoparasites on burrowing owls, ectoparasite levels could be manipulated to increase loads for some broods, while eliminating ectoparasites in others. This would control variation within and among levels of infestation and allow for easier comparison among treatment groups. However, because of the sensitive nature of burrowing owls, this type of manipulation was impractical in my study and thereby limited my experimental approaches in this experiment. To minimize direct impacts to burrowing owls resulting from habitat conversion to agriculture or development, mitigation efforts often attempt to provide burrowing owls with suitable habitat outside of a development area. Short-distance relocation of occupied nest burrows can be successful under certain circumstances. When compared to unsuccessful relocations, successful relocations in my study (1) were of shorter distance, (2) involved more adults with previous experience in the study area, (3) were associated
108 with less human disturbance after the relocation event, and (4) had one juvenile escape capture during the relocation process. Timing of nest relocation also was an important factor as delays in construction likely influenced two families to refuse occupancy of their relocated nests. The relocations I performed avoided the almost certain loss of several owl families that would have resulted from construction. However, this was a small study, restricted to a limited number of nests. Ideally, success rates for the techniques described here need to be quantified from much larger studies before such relocations can be performed where impacts cannot otherwise be avoided. Also, it remains completely unknown if the techniques I examined relate to owls nesting in natural burrows. Finally, effects of moving adults along with young on success rates of short distance relocations remain unknown. However, as in my study, it may be difficult to capture adults late in the nesting cycle, so timing of the relocation would be important. Hopefully, further research along the lines discussed here can help determine the efficacy of techniques that will reduce negative direct and indirect effects of land use change on burrowing owls.
