Non-Territorial Floaters in Great Horned Owls (Bubo Virginianus)
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Non-territorial Floaters in Great Horned Owls (Bubo virginianus) Christoph Rohner1 Abstract.—The ecology and behavior of non-territorial owls are basi- cally unknown. I studied the integration of young Great Horned Owls (Bubo virginianus) into the territorial breeding population from 1988- 1993 in the southwestern Yukon, Canada, during a peak and decline of the population cycle of snowshoe hares (Lepus americanus). Fifty- five fledglings were equipped with radio-transmitters that allowed weekly monitoring of individuals for 2-3 years. After a synchronized dispersal phase in each September, 29-45 percent remained within 35 km of their natal territories. Although 15 percent settled in a territory and were capable of reproducing before the end of their first year of life, most of these owls became non-territorial floaters. Sev- eral lines of evidence indicated that this behavior was caused by territorial exclusion of breeding pairs. Floaters were secretive and mostly resident within home ranges that were about five times the size of average territories. Movement patterns suggested that floaters were not involved in extra-pair matings, and that floating is not an alternative reproductive strategy. Survival of floaters was very high during peak densities of prey, leading to a proportion of 40-50 per- cent of non-territorial owls in the population. When numbers of snowshoe hares declined, emigration and mortality rates increased in floaters before territory owners were affected. The results of this study show how a large proportion of secretive floaters can delay the detection of population declines in traditional censuses of territorial birds, and can lead to serious underestimates of the impacts of predation. Non-territorial ‘floaters’, which live a secretive Very little is known about floaters in territorial life and form a ‘shadow population’, are well owl populations. known for some bird species and assumed for many others (Brown 1964, Newton 1992, Smith The question of why some birds in a population 1978, Watson and Moss 1970). Sometimes, do not establish a territory and do not breed such ‘surplus’ birds live in areas separate from has been approached from several directions. breeding territories, and they may become One hypothesis suggests that the social behav- directly observable when they form social ior of territory holders prevents them from groups (Birkhead et al. 1986, Charles 1972) or breeding (review in Newton 1992). Another they may be detectable in open habitat (Haller hypothesis suggests that a non-territorial stage 1996, Hannon and Martin 1996, Jenny 1992, in an individual’s life is not the fate of ‘doomed Watson 1985). Most of the knowledge about surplus’ birds, but is an alternative strategy floaters, however, is indirect and is derived leading to higher fitness than the strategy of from experimental removals of territory holders breeding early (Smith and Arcese 1989). Two (review in Newton 1992). The majority of owl elements could be involved in such a strategy: species are territorial, and ecological field (i) Life history theory predicts a trade-off be- studies are usually based on territorial birds. tween current investment and future survival, and delayed maturation may be particularly successful for long-lived species such as many 1 Research Associate, Centre for Biodiversity owls, because they would produce offspring Research, Department of Zoology, University of later in life when they are more experienced British Columbia, Vancouver BC, V6T 1Z4 and have more secure access to resources Canada. (Current address: Department of Renewable Resources, University of Alberta, 347 Edmonton, AB T6G 2H1, Canada.) 2nd Owl Symposium (review in Stearns 1992). (ii) Delayed establish- The goal of this paper is to present a portrait as ment of a breeding territory is not necessarily comprehensive as possible of floaters in a an inactive period in reproduction. Male selected owl species, and to encourage further floaters may gain extra-pair copulations with- studies on floaters in territorial owl popula- out the cost of defending a territory and provid- tions. ing the brood, whereas female non-territorial owls may secretively settle as a secondary mate METHODS of a territorial male of high quality (reviews in Birkhead and Møller 1992, Møller 1987, This study was part of a collaborative project Korpimäki 1988, Korpimäki et al. 1996). on the dynamics of the boreal forest ecosystem (Krebs et al. 1995). We worked at Kluane Lake How do floaters survive in a territorial owl (60˚ 57’N, 138˚ 12’W) in the southwestern population? Very little is known about the Yukon, and our study area comprised 350 km2 behavior of non-territorial owls. How vagrant of the Shakwak Trench, a broad glacial valley are they? Do they overlap in their space use bounded by alpine areas to the northwest and with territorial owls or are they restricted to southeast. The valley bottom averages about undefended habitat? Do they have special 900 m above sea level and is covered mostly behaviors to avoid aggression by territory with spruce forest (Picea glauca Blake), shrub owners, and how dangerous it is to intrude into thickets (Salix L. spp.), some aspen forest defended space? What is the foraging behavior (Populus tremuloides L.), grassy meadows with of non-territorial owls, where do they obtain low shrub (Betula glandulosa Raup.), old their food, and how do their intake rates com- burns, eskers, marshes, small lakes, and pare to territory owners? ponds. Finally, the question of how many floaters live The population data of Great Horned Owls in a territorial owl population arises. Because span the years 1988-1993, while most other territorial owls are easier to detect than float- data are from 1989-1992. Great Horned Owls ers, most ecological studies on owls are re- were censused in late winter and early spring stricted to the territorial fraction of a popula- on a 100 km2 plot within the main study area. tion. The consequences of varying floater Individual pairs were identified when hooting populations are particularly relevant to preda- simultaneously with neighbors at dawn and tion studies, which may underestimate the dusk, and obvious disputes between hooting effects of owls as predators, and to conserva- males or pairs were used for the mapping of tion studies, because a pool of non-territorial territorial boundaries. When necessary, play- birds can affect the recovery of populations backs of calls were used to elicit territorial (Newton 1991) or can mask population declines responses of owners and their neighbors. Most when census data is based on breeding territo- males were individually known, not only be- ries (Franklin 1992, Wilcove and Terborgh cause of radio-tagging but also because of their 1984). distinctly different hoots. These differences were later verified with sonograms from record- I studied Great Horned Owls (Bubo virginianus) ings at the nest (unpubl. data, method as used in the boreal forest in the southwest Yukon, for Strix aluco by Galeotti 1990). Observations Canada. Great Horned Owls are large, long- of territorial activity were made almost daily lived predators feeding mainly on lagomorphs from early February until late April (at least (Donazar et al. 1989). They are territorial year- 300 hours in each year). round, and are widely distributed across North and South America (Voous 1988). Occasional Survival estimates and information on move- irruptions of Great Horned Owls into southern ments were based on individual Great Horned Canada and the northern United States are Owls monitored by radio-telemetry. Twenty- linked to the decline phase in the 10-year one territorial adult owls were captured with population cycle of snowshoe hares (Lepus mistnets and cage-traps, and 55 owlets were americanus Erxleben), which is synchronized equipped with radio-transmitters before fledg- across boreal Canada and Alaska (Adamcik et ing (breakdown of sample sizes in Rohner al. 1978, Houston 1987, Houston and Francis 1996, 1997). Successful dispersers were later 1995, Keith and Rusch 1989, McInvaille and monitored intensively (3 hatched in 1988, 11 in Keith 1974, Rusch et al. 1972). 1989, and 16 in 1990), and 9 remained as non- territorial floaters in the study area. The radios 348 weighed 50 g including a shoulder harness of All arithmetic means are reported with stan- teflon ribbon for attachment as a backpack (< 5 dard errors and all probabilities are two-tailed percent of body weight, Kenward 1985). Bat- unless otherwise specified. Correlation coeffi- tery life was 2-2.5 years. The radios were cients were calculated as Spearman rank equipped with a two-phase activity switch correlations. For statistical testing, non- (sensitive to movement and change of angle). parametric tests were used wherever possible. The testing of bootstrap hypotheses followed All floaters and territory holders with transmit- the guidelines of Hall and Wilson (1991), and ters were normally monitored once per week two-sided probabilities were derived from 500 (for the presentation of weekly data, locations simulations (see also Rohner 1996). in addition to the weekly sampling intervals were excluded). Most checks were conducted RESULTS with hand-held equipment from the Alaska Highway, which follows the valley bottom for Dispersal of Juveniles and Age at Maturity the whole length of the study area. In addition, the entire area and its surroundings were Juvenile owls stayed in their natal territories searched for radio signals from helicopter or until September, and then rapidly dispersed in fixed-wing aircraft at least twice per year (in fall the following weeks (table 1). Dispersal dates after dispersal, and in spring after the onset of were delayed when the cyclic population of breeding). snowshoe hares started to decline in 1991 (U = 152, p = 0.01). By the end of the first week in Telemetry locations were obtained by triangu- October 1989 and 1990, only 4 percent (1 of lating owls with hand-held equipment. Topo- 27) were still in their natal territories.