WHY DO TERRITORIAL-MALE TENGMALMS OWLS FAIL TO OBTAIN A MATE

Non-breeding may occur because non-breeders are immature or somehow ph ysiologically incapable of breeding, or because of a lack of resources (e.g. food resources, mating partners) needed to breed. There is, how ever, a lack of experimental evidence on whether bachelor males posses sing territories and nest-sites are able to breed when supplemented wi th extra food or provided with mating partners. In vole-eating Tengmal m's owl, Aegolius funereus, we provided supplementary food and transfe rred females in nest-boxes of non-breeding males. Bachelor males that we supplemented with food did not attract mates at a higher frequency than unfed control males, which suggests that a lack of food did not i nfluence the ability to attract a mating partner. In contrast, bachelo r males presented with a female seemed to breed more frequently than b achelor males in the control group without mate addition. This suggest s that scarcity of females may be an important reason for the high pro portion of nonbreeding males in the population (c. 25%) and excludes t he possibility that non-breeding males are physiologically unable to b reed. The operational sex ratio of the owl population at the time of m ating may be male-biased, and some males may thus remain unpaired. Hab itat and nest-box quality also seemed to be lower among bachelors than among breeding males.