HABITAT UTILIZATION, DIET AND REPRODUCTIVE SUCCESS IN THE KESTREL IN A TEMPORALLY AND SPATIALLY HETEROGENEOUS ENVIRONMENT

Both habitat selection and foraging theory suggest that animals should choose environments in which their reproductive success is maximized and survival costs are minimized. We examined the habitats selected fo r foraging by Kestrels Falco tinnunculus breeding in western Finland d uring 1989-1991 and related this to prey availability and the breeding success. Voles (Microtus epiroticus and M. agrestis) were the main pr ey of Kestrels in our study area. Voles went through a three year popu lation cycle, with a crash in 1989, a low in 1990 and an increase in 1 991. We found considerable variation among years in the use of the mai n habitat, agricultural fields. Kestrels hunted mainly over fields in 1989 and 1991, whereas in 1990 they hunted mostly over forests and mar shland. The proportion of agricultural fields used as hunting habitat remained constant throughout the breeding season, except in 1989 when vole populations crashed and falcons shifted to hunt away from fields. Overall, reproductive success seemed to be better in small farmland a reas (size 0.1-10 km(2)) than in a large farmland area (100 km(2)). We conclude that small farmland areas contained suitable habitats for al ternative prey (bank voles, shrews, small birds, lizards, insects, etc .). Small farmland areas are thus probably more stable environments fo r Kestrels, because even in years of few Microtus voles in agricultura l fields there are favourable food patches closer to the nests than in the large farmland area.