DO BREEDING NOMADIC AVIAN PREDATORS DAMPEN POPULATION FLUCTUATIONS OF SMALL MAMMALS

The mean consumption of all prey by adult and young European kestrels (EKs), short-eared owls (SOs) and long-eared owls (LOs) in one breeding season was (+/- S.D.) 585 +/- 525 kg in 47 km2 of farmland area in western Finland during 1977-87. The proportion of prey consumption was highest by EKs (50%), followed by SOs (36%) and LOs (14%). The voles (Microtus agrestis and M. epiroticus) were the most frequent prey taken by the three species of birds of prey, though in vole lows shrews and small birds were the most abundant prey. In good vole years, Microtus voles suffered from a heavier predation than bank voles and common shrews, but the contrary was true in poor vole years when numbers of birds of prey were low. The pooled predation rate of Microtus voles by EKs, SOs and LOs was positively density-dependent. This indicates that they were able to dampen the amplitude of the vole cycle because they took a higher proportion of the summer standing crop and production of voles with increasing prey density and productivity. There was a negative correlation between the yearly vole consumption rates by avian predators and density changes of microtines from spring to autumn in farmland. When birds of prey removed > 500 voles per km2, microtine densities did not increase during the summer. Most individuals of all the three predatory species emigrate when voles crashed. The remaining ones shifted to alternative foods, made possible by a rich supply of alternative prey. Therefore, these predators were unable to deepen and prolong vole troughs.