The numerical response of great horned owls to the snowshoe hare cycle: Consequences of non-territorial 'floaters' on demography

1. The numerical response of great horned owls (Bubo virginianus Gmelin) to the 10-year population cycle of snowshoe hares (Lepus americanus Erxleben) in the boreal forest was examined during 1988-93 in the south-western Yukon, Canada. Demographic parameters were estimated based on censuses (territorial pairs), nest visits (productivity), and radio-telemetry (survival, emigration, and integration of young birds into the population). 2. Hares rose to peak densities in 1990, and almost all resident owl pairs bred and raised large broods during years of increasing and highest prey abundance. In 1991, the first year of hare decline, all breeding parameters including post-fledging survival were reduced, and recruitment in autumn was very low. In 1992 and 1993, reproduction was completely suppressed. 3. Survival of young owls in their first 2 years of life was high during the peak of the hare cycle, and a large number of non-territorial 'floaters' were present. These birds were silent, and moved more than territorial owls. Their ranges overlapped broadly with defended territories, and floaters were affected by the hare decline before territory holders. 4. Most ecological studies on birds are based on the territorial fraction of a population. 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.