Diet of breeding Tengmalm`s Owls Aegolius funereus: long-term changes and year-to-year variation under cyclic food conditions

The diet of breeding Tengmalm's Owls Aegolius funereus was studied in the Kauhava region, western Finland, using two methods: identification of prey animals cached in the nest-holes (a total of 4366 prey items in 1973-86) and found in the pellets and other prey remains collected from the nest-holes (12540 in 1966-86) . Microtus voles (M. agrestis andM. epiroticus) werethe mostabundant prey group in the caches (45% of the number of prey), followed bybank voles (32%), shrews (15%) and birds (5%) . Shrews amounted to 33% of prey items in the pellets, followed by Microtus spp . (27%), bank voles (24%) and birds (12%). During 1966-86, the percentages of water shrew, water vole, house mouse, brown rat and insects in the diet seemed to increase, whereas the percentages of field and bankvoles seemed to decrease . These long-term changes were probably caused by habitat changes in the study area. The proportion of Microtus voles (preferred prey) in the diet correlated positively with the abundance of these voles in spring trappings . The percentages of shrews and birds in the diet varied inversely with the numbers of Microtus spp . i n the food . This supported the prediction of the optimal foraging theory in the sense that the diet composition depended only on the density of preferred prey . Adult males are resident in the study area . Their preferred prey showed 3-4-year population cycles, but the populations of alternative prey (bank vole, common shrew and birds) did not show marked year-to-year fluctuations . Thus, resident males can survive by switching to alternative prey in low vole years