Home-Range Characteristics of Spotted Owls in ...

Home-Range Characteristics of Spotted Owls in Northern Arizona Author(s): Joseph L. Ganey and Russell P. Balda Source: The Journal of Wildlife Management, Vol. 53, No. 4 (Oct., 1989), pp. 1159-1165 Published by: Wiley on behalf of the Wildlife Society Stable URL: http://www.jstor.org/stable/3809627 . Accessed: 18/02/2015 18:21 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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OF SPOTTEDOWLS HOME-RANGE CHARACTERISTICS IN NORTHERN ARIZONA JOSEPHL. GANEY,Departmentof BiologicalSciences, NorthernArizonaUniversity,Flagstaff,AZ86011 RUSSELLP. BALDA,Departmentof BiologicalSciences, NorthernArizonaUniversity,Flagstaff,AZ 86011

Abstract: We studiedhome-rangecharacteristicsand movementsof Mexicanspottedowls (Strixoccidentalis lucida) in northern Arizona. Mean home-range size was 648 ha for 8 individuals and 847 ha for 3 mated pairs. Pair home ranges contained a mean of 403 ha of old-growth forest. Home-range size was positively correlatedwith elevation, amount of old-growthforest,and percentageof old-growthforest within the home range. Topographydid not directly constrainhome-rangesize, but did influencelocation of activity centers; i.e., areas that received heavy and repeated use by owls. All owls had activity centers located in old-growth forests on steep slopes, and visited other portionsof the home range infrequently. On average, 60 and 80% of individual locations fell within only 21 and 47%,respectively, of a home range. Two owls left the study area from November to April and >3 others remained on or near their summer range throughoutthe year. Owls showed seasonalshifts in use areas with the net result that year-roundhome ranges were larger than the areas used during any 1 season. J. WILDL. MANAGE.53(4):1159-1165

The spotted owl is most common in old-growth coniferous forests in the Pacific Northwest (Gould 1977, Marcot and Gardetto 1980, Forsman et al. 1984). Owl populations appear to be declining throughout this region as timber harvest reduces the amount of old-growth forest available (Gould 1977, Forsman et al. 1984). Little is known about the habitat requirements of the Mexican subspecies of the spotted owl, which inhabits rocky canyonlands and forested highlands in the southwestern United States and Mexico (Ganey et al. 1988). The Mexican subspecies is found primarily in unlogged mixedconifer forest in Northern Arizona, and could be influenced by timber harvest in this region (Ganey and Balda 1989). Information on habitat requirements of this subspecies is needed to evaluate the potential effects of timber harvest on owl populations and develop guidelines for habitat management. We report on home-range size, habitat composition within the home range, spatial use patterns, and seasonal movements of spotted owls in northern Arizona. We thank J. Stephenson for assistance in radiotracking owls; his careful work and attention to detail under difficult field conditions was greatly appreciated. N. L. Dodd and D. Aubuchon assisted in trapping and tracking owls in the White Mountains, and W. L. Eakle assisted in trapping. L. C. Ganey spent many hours analyzing home-range data. Radio transmitters and receivers were provided by D. R. Patton, T. G. Grubb, and G. C. Bateman; C. N. Slobodchikoff provided the home-range program.

Comments by G. C. Bateman, P. R. Krausman, and 2 anonymous reviewers improved the paper. This study was supported by the U. S. Forest Service (USFS), Southwest Region, through an agreement with the Arizona Game and Fish Department, Nongame Branch. We thank E. L. Fisher and R. L. Glinski for administering this agreement. Additional funding was provided by the National Wildlife Federation, the Arizona Wildlife Foundation, and the Arizona Wildlife Federation.

STUDYAREA We conducted radio-telemetry studies of spotted owls on 3 areas in northern Arizona: the San Francisco Peaks (SFP), Walnut Canyon (WC), and White Mountains (WM) study areas. The SFP were located 3 km north of Flagstaff, in north-central Arizona. The area was characterized by steep mountain slopes and canyons. Elevations ranged from 2,190 to 2,930 m. Vegetation was predominantly coniferous forest, with ponderosa pine (Pinus ponderosa) forests dominant at lower elevations and a mixture of ponderosa pine and mixed-conifer forests at higher elevations. Mixed-conifer forests were dominated by Douglas-fir (Pseudotsuga menziesii) and/or white fir (Abies concolor), often with a prominent component of ponderosa, limber (P. flexilis), and/or southwestern white (P. strobiformis) pines. Gambel oak (Quercus gambelii) was often present in the understory of both forest habitats. The WC was located 4 km southeast of Flag-

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staff. Walnut Canyon was a steep, narrow canyon eroded into a predominantly flat mesa. Rocky cliffs were prominent throughout the canyon, and several caves were present. Elevations ranged from 1,830 to 2,160 m. Vegetation within the canyon was diverse; mixedconifer forest dominated the north-facing slope, and south-facing slopes contained a mixture of ponderosa pine forest and pinyon (Pinus spp.)juniper (Juniperus spp.) woodland. The canyon bottom contained a mixture of coniferous and broadleaved trees, with extensive groves of boxelder (Acer negundo). The surrounding uplands were covered by selectively-logged ponderosa pine-Gambel oak forest, with numerous pinyon pines and junipers in the understory. The WM were located approximately 27 km southwest of Alpine, in eastern Arizona. This area was characterized by deep, steep-walled, heavily-forested canyons. Elevations ranged from 2,130 to 2,650 m. Vegetation was a mixture of mixed-conifer and ponderosa pine forests. All 3 study areas were characterized by cold, snowy winters and cool, rainy summers. These areas included the opposite extremes of spotted owl habitat in northern Arizona, ranging from mid-elevation rocky canyon habitat (WC) to high-elevation mountain slopes (SFP).

METHODS We captured spotted owls using a 3.4-m noose pole (Forsman 1983). Radio transmitters (AVM Inst. Co., Ltd., Livermore, Calif.) were attached using a backpack harness of 6-mm tubular teflon ribbon (Forsman 1983), and weighed 20-23 grams, including the harness. Radio signals were received using a TR-1 or TR-2 receiver and a hand-held 4 element Yagi antenna (Telonics Inc., Mesa, Ariz.). Diurnal locations were based on visual observation of roosting owls; nocturnal locations were determined by triangulation of compass bearings from known map locations. We conducted ac-3 tests on the study areas to construct error curacy polygons around triangulated locations (Springer 1979). Distance to the radioed owl greatly affects the size of the error polygon, because error arcs continue to diverge as distance increases. In most instances we were able to remain close enough to the owl so that the error polygons were 0.05), or between home-range size of males and females (Mann-Whitney U = 21.0, P > 0.05). Home-range size was positively correlated with mid-point elevation on the home range (r, = 0.87, n = 5, P < 0.05). Mean home-range size of paired owls was 847 ha (Table 3). On average, 66% of a pair's home range was used by both pair members, while the remaining 34% was used by only 1 pair member. Areas of overlap included the nest area (where known), primary roost areas, and the areas used most intensively for foraging. Because home-range estimates are influenced by sample size (Jennrich and Turner 1969), we computed area-observation curves (Odum and Kuenzler 1955) for all owls. We assumed that samples were adequate for delineation of homerange size when these curves reached an asymptote. In general, 125-150 locations collected over 20-30 nights in >1 season were needed to determine 85-90% of an owl's home range. Sample sizes for all owls met these requirements. Also, home-range size was not significantly correlated with sample size (r, = -0.55, n = 8, P > 0.05). Therefore, we concluded that the differences in home-range estimates among owls were not an artifact of sample size. These estimates do not represent year-round home ranges in all cases, however. Only 3 owls

J. Wildl. Manage.53(4):1989

were consistently tracked year-round; for the other 5 owls these estimates represent areas used during part of the year.

Withinthe HabitatComposition HomeRange Habitat composition within the home range varied among owls. Unlogged old-growth forests covered a mean of 43% of home ranges (range = 23-68%), with 55% covered by selectivelylogged forests. The mean amount of old growth contained within the home range was 310 ? 215 (SD) ha for individuals (range = 67-629 ha) and 403 ? 298 ha for pairs (range = 126-718 ha). The amount of old-growth forest within the home range was positively correlated with homerange size (r, = 0.9, n = 5, P < 0.05), as was the percentage of old growth (r, = 0.87, n = 5, P < 0.05). The mean size of old-growth forest patches within home ranges was 272 ? 222 ha (range = 67-612 ha). Mean size of old-growth patches was not significantly correlated with home-range size (r, = 0.7, n = 5, P > 0.05).

SpatialUse Patterns All owls had activity centers within their home ranges. These areas were used heavily and repeatedly by owls, while other parts of the home range received limited use. Activity centers encompassed a relatively small portion of the home range. On average, 60 and 80% of an owl's locations fell within only 21 ? 7.1 and 47 ? 12.0% of it's home range, respectively (analysis based on HM estimates). Activity centers included the nest site, major roost sites, and intensively-used foraging areas. Location of activity centers appeared to be influenced by topography and past logging practices. Old-growth forests were largely restricted to steep canyon and mountain slopes, and these areas coincided with areas heavily used by owls. Topography did not appear to constrain overall size or shape of home ranges in this study, as home-range boundaries did not coincide with any recognizable topographic barriers.

SeasonalMovements Seasonal movements varied among owls. Two owls left the SFP study area from mid-November to early April. These owls represented 2 territories and both sexes. We were unable to locate the wintering areas of these owls despite


J. Wildl. Manage. 53(4):1989

RANGES OFSPOTTED OWLS* Ganey and Balda

an extensive aerial search. At 1 site the female was found dead (apparently killed by a predator) 1 day before the male left the study area. At a second site the female migrated and the male remained on the home range throughout the winter. On the WC study area, the radioed owls remained on their home range throughout the year except for a brief period in late December and early January. Both owls moved approximately 10 km down canyon at this time. They returned to their usual home range within 2-3 weeks and remained there through the rest of the winter. We did not include this movement in home-range estimates. Home-range data in the WM were not subdivided seasonally because tracking occurred principally between May and October. We located owls in the WM once in mid-February. During this time, both owls were on their summer home ranges. Because this was long before migrating owls returned to the SFP study area, we suspect that owls in the WM remained on or near their summer home ranges all winter. We cannot rule out the possibility that these owls left their normal use areas for part of the winter, however. Home-range size was not significantly different between seasons for 5 owls that were tracked during both seasons (Mann-Whitney U = 26.0, P > 0.05). Sample sizes were small for some owls during the nonbreeding season (f = 80 locations, range = 43-121). All owls showed seasonal shifts in use areas. Mean overlap between breeding and nonbreeding season ranges was 64.6 ?_ 15.1% (range = 39.8-77.1%). Because of these shifts, total home-range size was larger than the area used during any 1 season.

DISCUSSION Schoener (1968) hypothesized that homerange size should be positively correlated with body size in raptors. This does not appear to hold for owls. Home ranges of spotted owls in northern Arizona were smaller than home ranges of spotted owls in Oregon (Forsman et al. 1984), but were larger than most home ranges reported for the larger conspecific barred owl (S. varia) (Nicholls and Fuller 1987). Home-range size varied 3-fold among spotted owls in northern Arizona. This variation was not an artifact of sample size, and did not appear to be caused by differences in tracking periods. We may not have determined total home-range

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size for owls that were not tracked year round. However, the 3 owls that were tracked year round occupied 3 of the 4 smallest home ranges, and owls that were not tracked during the winter occupied some of the largest home ranges (Table 2). This suggests that the large variation in home-range size was not caused by failure to determine total home-range size for some owls, and that this variation may be even greater than reported here. We suspect that the observed differences in home-range size were due to differences in resource availability and/or habitat quality. Differences in prey availability may be particularly important in this respect, but we have no data on prey abundance on the study areas. Studies of the prey base in these areas will be required to evaluate this hypothesis. Vegetation types could also be important in determining habitat quality. Spotted owls in northern California and Oregon had larger home ranges where forests were heavily cut over (Solis 1983, Forsman et al. 1984). Forsman et al. (1984) suggested that owls were able to occupy such areas by increasing the size of their home range to include additional areas of older forest. This did not appear to be the case in northern Arizona, where the amount and the percent of oldgrowth forest were highest on the largest home ranges. This suggests that owls were not using larger home ranges merely to include a minimum amount of old growth. We do not intend to imply that a minimum amount of old growth is not required. Spotted owls probably require a minimum amount of old growth, but we were unable to determine this amount. Home-range size was positively correlated with elevation in northern Arizona. This relationship was based on a small sample size, however, and the issue was further confused by habitat differences among the study areas and unequal tracking periods for owls. Further studies will be required to clarify the relative influence of elevation and habitat on home-range size. All owls used portions of the home range intensively and used other areas infrequently. These activity centers may be particularly important to the owls. Activity centers were located where large contiguous patches of oldgrowth forests were found on steep slopes, again suggesting that old-growth forests are important to spotted owls in northern Arizona. All owls tracked showed seasonal shifts in use


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areas. Such shifts may indicate that different areas or habitats are important in different seasons. Spotted owls were also partially migratory in northern Arizona. They were apparently nonmigratory in northern California and Oregon (Forsman et al. 1984; C. Sisco and R. J. Gutierrez, Winter ecology of radio-tagged spotted owls on Six Rivers National Forest, Humboldt County, Calif., USFS, Eureka, Calif., unpubl. rep.,

140pp., 1984). Laymon (1985) reported down-

slope movements in the Sierra Nevada Mountains of California which allowed owls to winter below the level of persistent snow. Migrating owls in this study left before any real accumulation of snow, and returned while the ground was still covered by snow. The mate of 1 migrating owl remained on its home range throughout the winter, indicating that overwinter survival on the area was possible for >1 owl. We suspect that seasonal movements were at least partly due to changes in food resources, and the variation observed in magnitude and duration of seasonal movements also suggests a component of individual variability.

IMPLICATIONS MANAGEMENT The strong association between Mexican spotted owls and old-growth forests indicates that old-growth forests should be protected where spotted owls occur. We do not know how much old-growth forest is required to support a pair of spotted owls, and this may vary among areas. Until more data are available on habitat requirements of this owl, we recommend an approach to management that preserves future options. We suggest that areas managed for spotted owls in northern Arizona be provided with an amount of old-growth forest equal to 1 standard deviation above the mean observed in this study (701 ha). This amount should include an unbroken tract of old-growth forest equal to the mean size of old-growth patches plus 1 standard deviation (494 ha). Home-range boundaries did not coincide with distinct topographic features in northern Arizona, and such features should not be used to delineate management areas. Activity centers were generally located around nest sites or major roost sites, however. We recommend centering management areas around the nest site, where known, and around a major roost site used during the nesting season (Mar-Jun) if the nest site cannot be located.


Wintering areas of migrating owls should be identified and potential threats to such areas evaluated. Protection of winter habitat may be as important as protection of breeding habitat. Further studies should investigate the habitat requirements of Mexican spotted owls and factors that influence these requirements. Such studies should be conducted in different geographic areas, should include sites representing a variety of habitats across an elevational gradient, and should include the prey base.

CITED LITERATURE J. 1980. Practicalnonparametricstatistics.Seconded. JohnWiley & Sons,New York, N.Y. 493pp. 1980. Harmonic DIXON,K. R., ANDJ. A. CHAPMAN. mean measure of animal activity area. Ecology 61:1040-1044. E. D. 1983. Methods and materials for FORSMAN, locatingand studyingspottedowls.U.S. For. Serv. Gen. Tech. Rep. PNW-162. Pac. NorthwestFor. and Range Exp. Stn. 8pp. ANDH. M. WIGHT. 1984. , E. C. MESLOW, Distribution and biology of the spotted owl in Oregon. Wildl. Monogr.87. 64pp. GANEY,J. L., ANDR. P. BALDA.1989. Distribution and habitat use of Mexicanspotted owls in Arizona. Condor 91:355-361. R. P. BALDA,ANDR. W. , J. A. JOHNSON, SKAGGS. 1988. Status report: Mexican spotted owl. Pages 145-150 in R. L. Glinski, B. G. Pendleton, M. B. Moss, M. N. LeFranc, Jr., B. A. Millsap,and S. W. Hoffman,eds. Proc.southwest raptormanage.symp. and workshop.Natl. Wildl. Fed. Washington,D.C. GOULD,G. I. 1977. Distributionof the spotted owl in California.West. Birds 8:131-146. R. I., ANDF. B. TURNER.1969. MeaJENNRICH, surement of non-circularhome range. J. Theor. Biol. 22:227-237. LAIR,H. 1987. Estimatingthe location of the focal center in red squirrelhome ranges. Ecology 68: 1092-1101. S. A. 1985. General habitats and moveLAYMON, mentsof spottedowls in the SierraNevada. Pages 66-68 in R. J. Gutierrezand A. B. Carey, tech. eds. Ecology and managementof the spottedowl in the Pacific Northwest. U.S. For. Serv. Gen. Tech. Rep. PNW-185. Pac. Northwest For. and Range Exp. Stn. 119pp. 1980. Statusof MARCOT,B. G., AND J. GARDETTO. the spotted owl on Six Rivers National Forest, California.West. Birds 11:79-87. T. H., ANDM. R. FULLER.1987. TerNICHOLLS, ritorial aspects of barred owl home range and behavior in Minnesota.Pages 121-128 in R. W. Nero, R. J. Clark,R. J. Knapton,and R. H. Hamre, eds. Biology and conservationof northernforest owls. U.S. For. Serv. Gen. Tech. Rep. RM-142. Rocky Mtn. For. and Range Exp. Stn. 309pp.

CONOVER, W.



RANGES OF SPOTTEDOWLS*

E. P., AND E. J. KUENZLER. 1955. Measurement of territory and home range size in birds. Auk 72:128-137. T. W. 1968. Sizes of feeding territories SCHOENER, among birds. Ecology 49:123-141. ODUM,

SOLIS,D. M., JR. 1983. Summer habitat ecology of

radio-taggedspotted owls on Six RiversNational Forest, California.M.S. Thesis, Humboldt State Univ., Arcata,Calif. 122pp. SPRINGER, J.T. 1979. Somesourcesof bias and error in radio triangulation.J. Wildl. Manage.43:926935. STUWE,M., AND C. E. BLOHOWIAH. 1985. Micro-

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computer programs for the analysis of animal locations (MCPAAL).Conserv. and Res. Cent. Natl. Zool. Park. SmithsonianInst. Front Royal,

Va.20pp.

SWIHART, R. K., AND N. A. SLADE. 1985.

Testing

for independence of observations in animal movements. Ecology 66:1176-1184.

UNITED STATES FOREST SERVICE.

1984.

Wildlife

coefficients technical report. U.S. For. Serv., Southwest.Region. Albuquerque,N.M. Received 6 October 1988. Accepted 8 April 1989.

AND CAVITYNESTINGBIRDSIN SNAG AVAILABILITY SLASHPINEPLANTATIONS DARRELL LAND,'Departmentof Wildlifeand Range Sciences, Universityof Florida,Gainesville,FL32611 WAYNER. MARION,Departmentof Wildlifeand Range Sciences, Universityof Florida,Gainesville,FL32611 E. O'MEARA,2 TIMOTHY Departmentof Wildlifeand Range Sciences, Universityof Florida,Gainesville,FL32611

Abstract: We examined snag densities and stand characteristicson 20 north Florida slash pine (Pinus elliottii) plantations;cavity nesting bird densities were estimated on 15 sites and relationshipsexamined between bird and snag densities and stand characteristics.Snag densities ranged from 2.6 to 38.6 snags/ha (f = 10.6 ? 1.4 [SE])and increasedas number of snag clustersincreased(P = 0.004). Eleven cavity nesting bird species were found with a mean density of 43.4 ? 7.2 birds/km2.Ninety-six percent of variationsin cavity nestingbird densityand diversitywas explainedby regressionson variousstandand snag characteristics. Managementfor cavity nesters in pine plantationsshould involve increasingrotation age, limiting the size of a single-aged forest, avoiding the creation of forest islands,and retaining snags within the stand. J. WILDL.MANAGE. 53(4):1165-1171

The Southeastern Coastal Plain originally was characterized by old-growth, longleaf-pine (Pinus palustris) forest on >24 million ha (Croker 1979). Historical and recent trends toward conversion of these forests to short-rotation plantations managed for pulpwood production create a potential threat to hole-nesting bird populations. Because snags are more abundant in natural stands, the long term impact of intensive pine management is decreased snag availability (McComb et al. 1986). Population densities of cavity nesters are positively related to snag density (Cunningham et al. 1980, O'Meara 1984, Raphael and White 1984). As a result, lower

densities of cavity nesting birds have been identified as the most important factor causing lower total bird density in managed conifer stands versus natural stands (Haapanen 1965). Management of existing forests to compensate for habitat changes resulting from more intensive forest management will require understanding the determinants of snag densities and characteristics in pine plantations and their value to cavity nesters. To evaluate some of these concerns, we tested 2 hypotheses: snag densities are not related to certain forest characteristics and cavity nesting bird densities are not related to certain forest characteristics, including snag densities. We thank L. D. Harris and F. E. Putz for I Present address:Florida Game and Fresh Water their input and assistance with various aspects Fish Commission,566 Commercial Boulevard, Na- of this research. K. M. Portier, P. V. Rao, and ples, FL 33942. C. T. Moore provided statistical advice. We also 2 Present address:Florida Game and Fresh Water Fish Commission,620 S. MeridianStreet,Tallahassee, thank D. E. Capen, R. N. Conner, W. R. ManFL 32399-1600. nan, D. Runde, and 3 anonymous reviewers for
