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Anirn. Behav., 1992, 43, 57 65

Dispersal restlessness: evidence for innate dispersal by juvenile eastern screech-owls? G A R Y R I T C H I S O N * , J A M E S R. B E L T H O F F t & E A R L J. SPARKS* *Department of Biological Scienees, Eastern Kentucky University, Richmond, K Y 40475, U.S.A. t Department of Biological Sciences, Clemson University, Clemson, SC 29634, U.S.A.

(Received 10 December 1990; initial acceptance 11 February 1991; final acceptance 29 April 1991; MS. number: a5936)

Abstract. Proximate factors responsible for the initiation of natal dispersal are poorly understood. Parental aggression, one possible factor, does not appear to initiate natal dispersal in young eastern screech-owls, Otus asio. Instead, intrinsic or innate factors may influence dispersal, resulting in increased activity near the time of dispersal. To examine the role of endogenous factors, screech-owl nestlings (approximately 15 days of age) were isolated and their activity levels monitored with digital pedometers for 20 weeks. Activity levels of captive owls ( N = 5) typically increased prior to the time they would have dispersed if not taken into captivity, then decreased following this time. Similarly, activity levels of freeliving, radio-tagged juveniles ( N = 3) increased until shortly after dispersal, then decreased during the postdispersal period. Captive owls gained mass for several weeks following their estimated fledging date, and then, despite ad libitum feeding, either sustained or lost mass around the time when dispersal normally occurs in free-living birds. Captive owls again gained mass during the post-dispersal period. Reduced mass near the time of dispersal may be adaptive in that lighter birds may be more competent in flight and, therefore, more capable of dispersing. The effects of extrinsic factors other than parental aggression are currently unknown, but these results suggest that natal dispersal in eastern screech-owls is influenced by intrinsic factors and appear to support a 'dispersal restlessness' model for the initiation of natal dispersal.

Natal dispersal occurs in virtually all birds and mammals prior to first reproduction. Much has been written concerning the ultimate or evolutionary causes of natal dispersal and, particularly, of sex-biased natal dispersal (e.g. Gauthreaux 1978; Greenwood 1980; Greenwood & Harvey 1982; Pusey 1987). The proximate factors responsible for natal dispersal have also been the subject of much conjecture. Howard (1960) suggested that dispersal may be of two types: (1) innate dispersal, in which animals are predisposed at birth to disperse beyond the confines of their parental home range, ignoring available and suitable areas and dispersing into strange and sometimes unfavourable habitats, and (2) environmental dispersal, in which an animal moves away from its place of birth in response to factors such as the absence of, or competition for, suitable resources, or parental aggression. In environmental dispersal, some dispersing individuals might be expected to experience aggression 0003-3472/92/010057 + 09 $03.00/0

prior to their departure, and such aggression has been reported in several species of birds. Bunn et al. (1982) suggested that the youngest barn owls, Tyto alba, in a brood are forced from the natal territory by the growing aggression of adults. Holleback (1974) reported that the breakup of black-capped chickadee, Parus atricapillus, broods may be the result of parental aggression toward the young or of aggression of the young toward each other. Aggression from parents or other adults has also been implicated in the natal dispersal of young mammals (see Moore & Ali 1984; Marks & Redmond 1987). Natal dispersal in many species occurs at a particular time of year, or when individuals reach a particular body mass, even in the absence of obvious extrinsic or environmental factors. For example, several species of birds and mammals disperse despite the apparent absence of aggression. Young marsh tits, Parus palustris, initiated dispersal from natal territories in spite of the removal 9 1992 The Association for the Study of Animal Behaviour 57

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Animal Behaviour, 43, 1

of one or both of their parents (Nilsson 1990). Furthermore, no evidence of parental aggression initiating natal dispersal has been reported in chaffinches, Fringilla coelebs (Marler 1956), great tits, Parus major (Royama 1962; Saitou 1979), or northern harriers, Circus cyaneus (Beske 1982). Similarly, in a review of dispersal in grounddwelling sciurid rodents, Holekamp (1984, page 309) concluded that 'male dispersers are not actively driven from their natal areas by conspecifics'. Natal dispersal also occurs in the absence of aggression in several species of primates (Pusey & Packer 1987). Such results indicate that natal dispersal may be effected by extrinsic factors other than aggression, or have innate or intrinsic components. Belthoff & Ritchison (1989) examined the natal dispersal of eastern screech-owls, Otus asio, in central Kentucky and found that young ( N = 16) dispersed from natal territories an average ( + SE) of 55 + 1 days after fledging. All young owls (presumably both males and females) dispersed during a 2-week period in mid-July. Although the effects of other extrinsic factors (e.g. population density and resource availability) on eastern screech-owl dispersal are currently unknown, Belthoff& Ritchison (1989) observed no evidence that parental aggression forced young to disperse. The apparent absence of parental aggression, and the tendency to disperse during a particular period, suggest that natal dispersal in eastern screech-owls has an intrinsic component. The natal dispersal of male Belding's ground squirrels, Spermophilus beldingi, is triggered by endogenous factors that increase the frequency of movement by juvenile males at the time of dispersal (Holekamp 1986). This suggests that natal dispersal may coincide with a period of restlessness, analogous to that exhibited by migratory birds. We sought to determine whether increased locomotor activity, triggered by endogenous factors, occurs in young eastern screech-owls near the time of dispersal using birds that had been isolated from extrinsic factors (population density, social infuences and food availability) since they were nestlings. Eastern screech-owls are particularly well-suited for such a study because (1) they survive well in captivity, (2) the number of days between fledging and the initiation of dispersal in freeliving individuals is known (Belthoff & Ritchison 1989), and (3) in contrast to many smaller birds (e.g. passerines) this period is sufficiently long

(approximately 8 weeks) for trends in activity levels to appear. MATERIALS

AND METHODS

Captive Studies of Activity We removed six nestling eastern screech-owls from three nest cavities (two young/nest) at the Central Kentucky Wildlife Management Area situated 17 km southeast of Richmond, Madison County, Kentucky. Because nestling screech-owls cannot be sexed based on external morphology or other features, we randomly removed nestlings. We hoped to sample members of both sexes. We determined the sample size (N= 6) based on the number of isolation chambers that were available for use (see below). We removed owls from nests between 7 and 10 May 1989, at which time all owls appeared to be about the same age based on their development (about 15-20 days old or 1 2 weeks from fledging). In the laboratory, owls were kept in individual wire-mesh cages measuring 0'5 x 0'5 • 0-3 m, placed in individual isolation chambers measuring 1.5 x 1.5 x 1.5 m. Owls were prevented from interacting visually or vocally. We provided food (laboratory mice, chicken gizzards and hearts) and water ad libitum. The owls were kept on a natural photoperiod, and the temperature was maintained at 23.5_+ 2.0~ We began recording activity levels on 18 May, our estimated date that owls would probably have left their respective nests. Owls in the same study area had a mean fledging date (i.e. when young left nest cavities permanently) of 21 May (for seven families of screech-owls during 1985 and 1986, range= 14-30 May; Belthoff & Ritchison 1989). We continued monitoring activity levels until 6 October, by which time the initial dispersal movements of young screech-owls in central Kentucky generally concluded (Belthoff & Ritchison 1989). After the experiment, we killed (U.S. Fish & Wildlife Service Permit no. PRT-745253) and sexed the owls. By chance, each pair of siblings included a male and a female. We used Micronta Walk-Mate pedometers (Cat. No. 63-671, Radio Shack, Fort Worth, Texas) to monitor activity levels of captive screech-owls. These digital pedometers register steps or 'hops' (i.e. individual up-and-down movements) rather than distance and, therefore, provide accurate information concerning the number of movements

Ritchison et al.: Dispersal restlessness in owls owls made. Shivering, preening or other minor adjustments in body position did not register on the pedometers. The pedometers originally weighed about 30 g but excess plastic housing material was trimmed to reduce their weight. We attached pedometers to owls back-pack style with woven nylon cord (see Smith & Gilbert (1981) for backpack attachment of radio packages). Complete packages, pedometers plus nylon cord, weighed an average of 18 g. At the beginning of the experiment (18 May), the six owls ranged from 105-131 g (,~= l19g). Initially, therefore, the packages ranged from about 14-17% of the owls' body masses. After 4 weeks, the owls ranged from 131-161 g (.~= 150 g) and, thus, the pedometer packages ranged from about 11-14~ of the owls' body masses. We read each owl's pedometer daily at about 1500 hours. After recording the number of movements, we reset pedometers to zero. Because pedometers were read only once per day, we were unable to determine whether most of the activity occurred during the night or day. We also weighed owls to the nearest 0-5 g every 3 days. We examined differences in the activity levels of owls over time by pooling activity data into 7-day periods ( N = 7 per week for each owl). We used repeated-measures ANOVAs to examine the effects of time (i.e. week) and sex on activity level and mass. If significant effects existed, we estimated least squares means and compared these means using pairwise t-tests. Field Studies of Activity To determine activity levels of free-living juvenile eastern screech-owls prior to, during and following dispersal, we radio-tagged juveniles in two families (owls 867, 868 and 869 in family 1; owls 870, 871 and 872 in family 2) on the study area during May 1985 (see Belthoff 1987; Belthoff & Ritchison 1990a, b for descriptions of this area). We attached radio-transmitters (Wildlife Materials, Carbondale, Illinois) back-pack style with woven nylon cord (Smith & Gilbert 1981). These packages weighed less than 8 g, which was much lighter than pedometer packages to allow for flight. After young owls fledged, we tracked individuals by radio with portable receivers, hand-held two-element yagi antennae and hand-held compasses. Observation periods lasted 1-4 h (between 1800 and 0100 hours) for 36 weeks or until we could no longer locate

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individuals. During the pre-dispersal phase (i.e. the first 7 or 8 weeks after fledging), we attempted to track each family on two nights per week ( N = 4 nights per week). Following dispersal of young owls, we limited tracking to two nights per week. To determine the location of owls, we took bearings simultaneously from two tracking stations, synchronized by walkie-talkies (Smith & Gilbert 1984; Wijnandts 1984), and then analysed bearings with the TELEM program (Koeln 1980). The location of each owl was placed at the centre of the error polygon (Springer 1979). We recorded the time since last locating owls and used distance moved between successive locations on a given night of tracking as an index of activity level. We were able to monitor three juvenile owls (owls 867, 868 and 870) during both the pre-dispersal and post-dispersal periods. We tracked owl 867 (unknown sex) during 10 of the weeks, owl 868 (male) during 14 weeks and owl 870 (unknown sex) during 11 weeks. We obtained an average ( _ SE) of 6"0+0"3 locations per owl each night of tracking. The average time between successive locations was 37-2 min (range = 28-9-53.0 min). There was, however, a significant effect of time interval on distance moved between successive locations (F=2.65, df=27,367, P_

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Figure 2. Mean ( _ sE) weekly activity levels of captive male ( : N = 3) and female ~ : N = 2) juvenile eastern screech-owls in the weeks following fiedging. Arrow: mean date for initiation of dispersal in free-living screech-owls.

Animal Behaviour, 43, 1

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Figure 3. (a) Mean (_+ SE)weekly mass (g) for captive, juvenile eastern screech-owls (N= 5) following fledging. (b) Mean weekly mass for individual owls. : males; - - - : females. Arrows: mean date for initiation of dispersal in free-living screech-owls.

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Figure 4. Mean ( _ SE) distance between successive telemetry locations of three radio-tagged juvenile eastern screechowls for the weeks following fledging ([]: owl 867; I : owl 868; [~: owl 870). Values for weeks 19-36 post-ftedging have been combined in week 19+. No data were available for week I0. See text for mean sample sizes per night of radio tracking. Arrow: mean date for initiation of dispersal.

Ritchison et al.: Dispersal restlessness in owls

probably deposit fat and gain mass more rapidly than free-living owls. Migratory restlessness has been observed in a variety of bird species, and levels of activity may exhibit very obvious and significant peaks during this time (e.g. Berthold 1984). Although we found that activity levels of young screech-owls were high during the period of dispersal, some captive owls did not exhibit obvious peaks in activity. The absence of clear-cut peaks in activity may be related to the dispersal behaviour of eastern screech-owls. The initial dispersal movements of most young screechowls appear to be relatively short in both length and duration. For example, 17 young screech-owls in central Kentucky dispersed a median distance of 2.3 km, and at least seven individuals reached their eventual winter ranges within a week (Belthoff & Ritchison 1989). Berthold (1984, 1988) found a significant and positive relationship between the amount of restlessness and the distances covered during migration of 13 species of European warblers. Thus, if activity levels of juveniles are correlated with dispersal tendencies, it is possible that short-distance dispersers, like most young screech-owls, exhibit less locomotor activity than long-distance dispersers. Although activity levels of captive juveniles remained relatively high during the period of dispersal, all owls exhibited higher activity levels sometime during weeks 3, 4 or 5. As noted above, this may have resulted from a premature triggering of an ontogenetic switch. However, high activity levels during these weeks would probably be beneficial to owls in the field. That is, young owls are probably practising their flying and hunting skills during this period, and increased activity levels may enhance this process. Furthermore, young screechowls are apparently dependent on their parents for at least 5 weeks after leaving the nest (Belthoff 1987; Belthoff& Ritchison 1990a; Sparks 1990), and high activity levels during weeks 3-5 may help young owls maintain contact with their parents (and perhaps enhance their chances of being fed by parents) or increase their chances of out-competing siblings for the attention of parents. Activity levels of captive owls were typically lowest during weeks 1 2. Relative inactivity during this time may also be adaptive by making it easier for parents to locate and feed their young. Alternatively, relatively little activity by captive owls during the first 2 weeks may have resulted from the low mass of captive owls and the relatively greater burden (in terms of percentage

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of body mass) of the pedometer packages. Finally, as would be expected if the initiation and duration of natal dispersal were influenced by endogenous factors, the activity levels of all five owls declined after the dispersal period. Despite ad libitum feeding, the mass of captive owls either declined or levelled offprior to (weeks 5 or 6 post-fledging) or during the predicted dispersal period, and then began to increase several weeks later (week 11 or 12 post-fledging). Drent (1984) noted that young great tits lost 10% of their body mass in the days immediately preceding dispersal, and Sullivan (1989) reported weight loss in young yellow-eyed juncos, Junco phaenotus, after they became independent of their parents. Such fluctuation in mass is generally thought to be the result of inefficient foraging by juveniles following the termination of provisioning by parents (e.g. Sullivan 1989). However, Skutch (1976) speculated that the loss of appetite (and mass) reported in some young birds around the time of fledging might be adaptive because lighter fledgings would be more competent in flight. Norberg (1981) suggested that a bird whose mass decreases should exhibit enhanced flight performance (improved linear horizontal acceleration, horizontal flight speed, rate of climb and turning ability). Thus, a levelling offor decrease in body mass at the time of dispersal and for a period after dispersal may be adaptive. Lighter individuals could be more competent in flight and, therefore, better able to disperse. Furthermore, for those species, like owls, that rely on flight performance for foraging, enhanced flight performance would probably improve foraging success (Norberg 1981). This might be of critical importance for newly independent young owls foraging in new areas. A few weeks after dispersal (i.e. after acquiring a territory), increasing mass would be advantageous for young owls because fat deposits would improve their chances of surviving the winter (Henny & VanCamp 1979). Juvenile male screech-owls were significantly more active than females while in captivity. We found no published information on relative activity rates of dispersing males and females for other species of birds, but similar results were reported in at least one species of mammal. Captive juvenile male Belding's ground squirrels exhibited significantly greater levels of activity (i.e. more climbing and greater distance moved per hour of observation) then juvenile females; in the field, all males dispersed while most females were philopatric (Holekamp

Animal Behaviour, 43, 1

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1986). Unlike Belding's ground squirrels, both male and female eastern screech-owls disperse (VanCamp & Henny 1975; Belthoff & Ritchison 1989). If, as in Belding's ground squirrels, activity levels of captive juveniles are correlated with dispersal behaviour, our data suggest that male screech-owls may disperse greater distances than females. In fact, Gehlbach (1986) suggested that juvenile male screech-owls may have to disperse farther than females because males must seek out new, unoccupied terrain, but at least some females may be accepted as mates by older males possessing territories nearby. In conclusion, our results indicate that activity levels of both free-living and captive juvenile eastern screech-owls are relatively high in the weeks leading to dispersal, then decline during the post-dispersal period, and they generally support a dispersal restlessness model. To our knowledge, these results constitute the first evidence for a relationship between activity levels and the timing o f natal dispersal in birds. Our study also represents an attempt to develop methodology for the laboratory study of dispersal behaviour in birds. Certainly, comparative information from other species of birds would help clarify the general importance of the relationship between activity levels and the timing of natal dispersal. ACKNOWLEDGMENTS We thank Paul Cavanagh, Paul Klatt and Keith Krantz for assistance in the field and laboratory, and Patty Gowaty, Jack D u m b a c h e r and two referees for constructive comments on the manuscript. We also thank Sidney Gauthreaux, Jon Plissner, D a v e Tonkyn and other members of Clemson University's Behavioural Ecology Research G r o u p for discussions that stimulated this research. Funding for our research on screech-owls has been provided by Eastern Kentucky University and by Sigma Xi, The Scientific Research Society. We collected young owls under U.S. Fish & Wildlife Service Permit PRT-745253 and a Kentucky Educational Wildlife Collecting Permit. REFERENCES

Belthoff, J. R. 1987. Post -fledging behavior of the eastern screech-owl (Otus asio). M.S. thesis, Eastern Kentucky University, Richmond. Belthoff, J. R. & Ritchison, G. 1989. Natal dispersal of eastern screech-owls. Condor, 91,254-265.

Belthofl', J. R. & Ritchison, G. 1990a. Roosting behavior of postfledging eastern screech-owls. Auk, 107, 567-579. Belthoff, J. R. & Ritchison, G. 1990b. Nest-site selection by eastern screech-owls in central Kentucky. Condor, 92, 982-990. Berthold, P. 1984. The endogenous control of bird migration: a survey of experimental evidence. Bird Study, 31, 19-27. Berthold, P. 1988. The control of migration in European warblers. Proc. int. ornithol. Congr., 19, 215-249. Beske, A. E. 1982. Local and migratory movements of radio-tagged juvenile harriers. Raptor Res., 16, 39-53. Bunn, D. S., Warburton, A. B. & Wilson, R. D. S. 1982. The Barn Owl. Vermillion, South Dakota: Buteo Books. Drent, P. J. 1984. Mortality and dispersal in summer and its consequences for the density of great tits Parus major at the onset of autumn. Ardea, 72, 127-162. Gauthreaux, S. A., Jr. 1978. The ecological significance of behavioral dominance. In: Perspectives in Ethology. Vol. 3 (Ed. By P. P. G. Bateson & P. H. Klopfer), pp. 17-54. New York: Plenum Press. Gehlbach, F. C. 1986. Odd couples of suburbia: Nat. Hist., 6, 56-66. Greenwood, P. J. 1980. Mating systems, philopatry and dispersal in birds and mammals. Anita. Behav., 28, 1140-1162.

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Ritchison et al.: Dispersal restlessness in owls Pusey, A. E. & Packer, C. 1987. Dispersal and philopatry. In: Primate Societies (Ed. by B. B. Smuts, D. L. Cheney, R. M. Seyfarth, T. T. Struhsaker & R. W. Wrangham), pp. 250-266. Chicago: University of Chicago Press. Royama, T. 1962. A study of the breeding behavior and ecology of the great tit, Parus major. Ph.D. thesis, Tokyo University. Saitou, T. 1979. Ecological study of social organisation in the great tit, Parus major L. III. Home range of the basic flocks and dominance relationships of the members in a basic flock. Misc. Rep. Yamashina Inst. Ornithol., 11, 137 148. Skutch, A. F. 1976. Parent Birds andtheir Young. Austin: University of Texas Press. Smith, D. G. & Gilbert, R. 1981. Backpack radio transmitter attachment success in screech-owls (Otus asio). N. Am. Bird Bander, 6, 142-143.

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Smith, D. G. & Gilbert, R. 1984. Eastern screech-owl home range and use of suburban habitats in southern Connecticut. J. Field Ornithol., 55, 322-329. Sparks, E. J. 1990. The spatiotemporal ecology ofjuvenile and adult eastern screech-owls in central Kentucky. M.S. thesis, Eastern Kentucky University, Richmond. Springer, J. T. 1979. Some sources of bias and sampling error in radio triangulation. J. Wildl. Mgmt, 43, 926-935. Sullivan, K. A. 1989. Predation and starvation: agespecific mortality in juvenile juncos (Junco phaenotus). J. Anim. Ecol., 58, 275-286. VanCamp, L. F. & Henny, C. J. 1975. The screech owl: its life history and population ecology in northern Ohio. N. Am. Fauna, 171, 1 75. Wijnandts, H. 1984. Ecological energetics of the longeared owl (Asio otus). Ardea, 72, 1-92.