DIET COMPOSITION, PREY CHOICE, AND BREEDING SUCCESS OF LONG-EARED OWLS - EFFECTS OF MULTIANNUAL FLUCTUATIONS IN FOOD ABUNDANCE

Common voles (Microtus epiroticus) were the main prey of Long-eared Owls (Asio otus) breeding in western Finland during 1977 - 1989. They constituted, on average, 58 % (range < 15 to > 75 %) of the number of prey. Field voles (Microtus agrestis), common shrews (Sorex araneus), bank voles (Clethrionomys glareolus), and water voles (Arvicola terrestris) were the most frequent alternative prey. The owls showed strong preference for common voles over the alternative prey species and this preference increased with vole density. If the common vole is the most profitable prey of Long-eared Owls, the between-years variation in the diet was consistent with the three predictions of the conventional model of the optimal diet theory. (1) Predators should feed on the most valuable prey type when prey are abundant. There was a positive correlation between the abundance of common voles and their proportions in the food. (2) No relationships for alternative prey types agreed with the prediction that the inclusion of a prey type in the diet depends only on the abundance of the preferred prey. (3) As predicted by the optimal diet theory. the diet width expanded when the density of common voles decreased and shrank when vole density increased. Annual breeding density (range 0.0-0.4 nests/km2), mean clutch size (3.0-6.3), and mean brood size (0.0-3.5) were positively related to the spring abundance of common and field voles. Accordingly, it is adaptive for Long-eared Owls to breed in a good area for voles. Because vole abundances in Fennoscandia fluctuate markedly between years and asynchronously between areas, Long-eared Owls should stay in the same area in the increase phase of the vole cycle, but move away when voles decline.