DIFFERENTIAL AVIAN PREDATION ON SEX AND SIZE CLASSES OF SMALL MAMMALS- DOOMED SURPLUS OR DOMINANT INDIVIDUALS

Predators of small mammals may selectively kill either large individua ls, usually male or the old, or small individuals, often females and y oung. We studied the prey choice of breeding male Tengmalm's owls (Aeg olius funereus) in western Finland. The owls fed mainly on sibling vol es (Microtus rossiaemeridionalis), field voles (M. agrestis), bank vol es (Clethrionomys glareolus) and common shrews (Sorer araneus). We ide ntified, sexed and weighed prey items cached by the owls in their nest -boxes, and compared characteristics of these prey to small mammals tr apped in the same study area during 1985-1992. For each of the three v ole species, owls captured more males than females, but we did not fin d that owls preferred one sex of common shrews. Our long-term data ind icated that male-bias of two Microtus species in the diet of owls was highest in the low phase of the vole cycle, and decreased through the increase and peak phases. This suggests that the two sexes of voles be have differently, but that these differences change over the course of the 3-yr cycle. The proportion of prey individuals with a small body mass was greater in owl caches compared to trapping censuses, irrespec tive of species or sex. Large dominant individuals may occupy safe hab itats with dense vegetation cover, where avian predation risk is minim al.