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UBC Theses and Dissertations

The numerical response of great horned owls to the snowshoe hare cycle in the boreal forest Rohner, Christoph


Great homed owls (Bubo virginianus) are among the most opportunistic avian predators. In the subarctic boreal forest, their diet consists mainly of snowshoe hares (Lepus americanus), which show extreme population cycles with a 8-11 year period. The aim of this thesis was to study the population ecology of great homed owls in a cyclic environment, to investigate the components of the numerical response of this predator to its prey, and to evaluate the evolutionary context of the ecological processes involved. The study was conducted from 1989-92, with some data from 1988 and 1993, at Kluane Lake in the southwestern Yukon, Canada. During the increase phase of the snowshoe hare cycle, 86% of resident owl pairs bred and raised large broods of 2.4-2.6 fledglings per successful nest. Survival of young owls in their first two years of life was high, and two females were observed to breed as yearlings. Densities of territorial owls almost doubled to a maximum of 21-24 pairs/100km² from 1988-92, but most owls that recruited locally became non-territorial 'floaters', presumably because social behaviour limited the number of territories. Floaters were silent, their ranges overlapped with those of territorial birds, and their density reached 40-50% of the total population. Snowshoe hares began to decline in the winter of 1990/91, and the number of great homed owls recruited in fall dropped from 1.7/pair in 1989-90 to 0.3/pair in 1991. Proximate causes of high pre-dispersal mortality included predation by mammals and parasitism by black flies (Simuliidae) and by Leucocytozoon ziemanni, a blood parasite transmitted by these flies. Post-dispersal mortality and emigration of resident owls also increased as hare densities declined further, floaters being affected before territorial birds. Owl densities continued to increase after the hare peak and then declined with a time lag of one year for floaters, and two years for territory holders. Responses to brood size manipulations and food additions suggested that food was not super-abundant during the prey peak. I conclude that territorial behaviour is essential in causing time lags, with some birds monopolizing resources and conserving energy by ceasing reproduction. A review of life history variation in northern owls showed that great horned owls are constrained in their phenotypic plasticity to increase reproduction at high prey levels compared to some other species. Based on a comparison of evolution in the genera Bubo and Nyctea, I hypothesize that habitat-specific differences in the availability of prey during reproduction, and also in the mortality on age classes during bottlenecks, favoured the diversity of life histories in northern owls.

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