Body Mass of Breeding Tengmalm's Owls Aegolius funereus: Seasonal, Between-Year, Site and Age-Related Variation

Abstract

I measured wing lengths and weighed female Tengmalm's Owls at 404 nests during 1977-88 and males at 293 nests during 1979-88 in western Finland. Breeding females lost mass from the pre-laying and laying periods to the incubation stage and during the nestling period, whereas they kept constant mass during incubation. The body mass of breeding males did not change from the late incubation and hatching periods to the late nestling phase. The mean mass loss of breeding females was larger in poor vole years than in good ones. The yearly mean body mass of females was positively correlated with the spring trap index of Microtus and Clethrionomys glareolus, but this relationship was not significant for males. The wings of females were longer and bodies heavier on good territories than on poor ones, but territory quality did not affect the body mass of males. Intersexual differences in the seasonal and between-year changes in body mass were probably associated with different breeding duties of the sexes. Males do most of the hunting and cannot lay down large reserves that increase flight costs. Incubating and brooding females adopt the passive storage role; they are able to fast on average five days based on their body reserves at the start of breeding. The results of the study mainly support the adaptation hypothesis (the mass reduction of adult birds rearing young is an evolved adaptation), but the fact that females lost more mass in poor years than in good ones is better explained by the stress hypothesis (mass reduction is caused by food shortage and hard work).