Numerical and Functional Responses of Kestrels, Short‐Eared Owls, and Long‐Eared Owls to Vole Densities

Abstract

We studied numerical and functional responses of breeding European Kestrels (EK) (Falco tinnunculus), Short—eared Owls (SO) (Asio flammeus), and Long—eared Owls (LO) (Asio otus) during 1977—1987 in 47 km 2 of farmland in western Finland. The pooled mean yearly breeding density varied from 0.1 to 2.4 pairs/km 2 . The number of nesting EKs (range 2—46 pairs), SOs (0—49), and LOs (0—19) fluctuated in close accordance with the spring density of Microtus (M. agrestis and M. epiroticus) voles. The mean yearly number of fledglings produced per pair ranged from 0.4 to 3.8 and, for each species, was positively correlated with spring density of Microtus voles. Due to their high degree of mobility, EKs, SOs, and LOs were able to track the population fluctuations of their microtine prey without time lags. An increase in microtine densities caused a rapid immigration into the study area and a decrease caused a rapid emigration from the area. Microtus voles were the most important prey group by mass in the diet of each species. Water voles, bank voles, shrews, and small birds were the most frequent alternate prey. The spring density of Microtus spp. was positively correlated with the percentage of these voles in the diet of EK, SO, and LO. The pooled functional response curve of these three raptor species to the fluctuating densities of Microtus spp. was close to linear, indicating that consumption rates are independent of vole densities. Breeding EKs, SOs, and LOs seemed to take a larger proportion of voles available in peak years than in low ones.