Non-territorial Floaters in Great Horned Owls (Bubo virginianus)

—The ecology and behavior of non-territorial owls are basically unknown. I studied the integration of young Great Horned Owls (Bubo virginianus) into the territorial breeding population from 19881993 in the southwestern Yukon, Canada, during a peak and decline of the population cycle of snowshoe hares (Lepus americanus). Fiftyfive 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. Several 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 percent 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. 1 Research Associate, Centre for Biodiversity Research, Department of Zoology, University of British Columbia, Vancouver BC, V6T 1Z4 Canada. (Current address: Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada.) Non-territorial ‘floaters’, which live a secretive life and form a ‘shadow population’, are well known for some bird species and assumed for many others (Brown 1964, Newton 1992, Smith 1978, Watson and Moss 1970). Sometimes, such ‘surplus’ birds live in areas separate from breeding territories, and they may become directly observable when they form social groups (Birkhead et al. 1986, Charles 1972) or they may be detectable in open habitat (Haller 1996, Hannon and Martin 1996, Jenny 1992, Watson 1985). Most of the knowledge about floaters, however, is indirect and is derived from experimental removals of territory holders (review in Newton 1992). The majority of owl species are territorial, and ecological field studies are usually based on territorial birds. Very little is known about floaters in territorial owl populations. The question of why some birds in a population do not establish a territory and do not breed has been approached from several directions. One hypothesis suggests that the social behavior of territory holders prevents them from breeding (review in Newton 1992). Another hypothesis suggests that a non-territorial stage in an individual’s life is not the fate of ‘doomed surplus’ birds, but is an alternative strategy leading to higher fitness than the strategy of breeding early (Smith and Arcese 1989). Two elements could be involved in such a strategy: (i) Life history theory predicts a trade-off between current investment and future survival, and delayed maturation may be particularly successful for long-lived species such as many owls, because they would produce offspring later in life when they are more experienced and have more secure access to resources