Effect of Snow Cover on Seasonal Changes in Diet, Habitat, and Regional Distribution of Raptors That Prey on Small Mammals in Boreal Zones of Fennoscandia

Seasonal changes in spatial distribution of search effort of birds that prey on small mammals were studied in two structurally different coniferous forest habitats in the northern boreal zone in SE Norway. During the season with snow cover both the proportion of Microtus relative to that of Clethrionomys in the predators' diet, and their use of a clear-cut relative to that of older forest were lower than during the snow-free season. This was related to a lower relative availability of prey (Microtus agrestis and M. oeconomus) in the clear-cut when the ground was snow-covered than when it was snow-free. Based on this local pattern I suggest the following explanation for differences in migratory strategy between raptors that prey on small mammals in Fennoscandian boreal zones: species that migrate to snow-free areas in winter are either adapted to hunt by the energetically expensive method of quartering in open grassland habitats, where prey (Microtus) availability is relatively low during periods with snow cover (hen harrier Circus cyaneus, short-eared owl Asio flammeus, and long-eared owl A.otus), or by sit-and-wait in open grassland and forest habitats, the latter with relatively high prey availability during periods with snow cover, but unable to locate concealed prey (kestrel Falco tinnuculus, common buzzard Buteo buteo, and rough-legged buzzard B. lagopus). In contrast, species that remain in areas with permanent snow cover during winter use the energetically cheap sit-and-wait tactic, and are able to hunt in closed forest habitat and localize concealed prey (the remaining owl species). Interspecific differences in prey availability as determined by hunting habitat and hunting mode is probably more important in shaping the migration patterns of Fennoscandian owls than is nest site availability.