What determines the foraging distribution of raptors on heather moorland?

Abstract

Foraging theory predicts that mobile predators such as raptors should hunt more frequently in habitat patches yielding the greatest energy gains. Raptor foraging distributions may, however, be constrained by nest location and by competition between different raptor species. We tested these hypotheses in a moorland raptor assemblage including two vole specialists and three generalist predators. We studied spatial and temporal variation in the foraging distributions of these raptors during breeding and non-breeding seasons on an area of moorland managed for red grouse in Scotland. We compared raptor foraging distributions to indices of prey abundance and availability, the habitat characteristics of foraging sites, and for some species, the distance from nest sites. We also assessed whether the foraging distributions of the smaller raptors were influenced by the larger raptors. Short-eared owls Asio flammeus, kestrels Falco tinnunculus and common buzzards Buteo buteo showed pronounced temporal variation in foraging in either breeding or non-breeding seasons in response to fluctuations in the abundance of small mammals. Having removed year effects, however, spatial variation in foraging by these species within and outside the breeding season was not explained by small mammal abundance or availability. In contrast, the foraging distributions of hen harriers Circus cyaneus and peregrines Falco peregrinus showed no significant temporal variation. During the breeding season, male and female harriers foraged on areas near harrier nests. Outside the breeding season, female harriers foraged in areas with more grouse and small mammals. Peregrines showed no foraging response to prey during the breeding season but during the non-breeding season they hunted in areas with more grouse. There was little evidence that the foraging distributions of smaller raptors were influenced by larger raptors; however, golden eagles Aquila chrysaetos were rarely seen at our study site. These results suggest the simple dichotomy between specialist and generalist predators that masks a more complex response to prey abundance.