Effects of cyclic food supply on breeding performance in Tengmalm's owl (Aegolius funereus)

Abstract

Cyclic populations of bank voles (Clethrionomys glareolus), grey-sided voles (Clethrionomys rufocanus), and field voles (Microtus agrestis) made up > 90% of the diet of Tengmalm's owl (Aegolius funereus) in northern Sweden during the breeding seasons of 1980–1986. These voles also made up > 90% of snap-trapped small mammals in the study area. Comparisons of the species composition in the diet during laying with that of animals trapped indicated that bank voles were usually taken approximately in proportion to their relative abundance, whereas field voles were frequently taken more and grey-sided voles less than expected from their relative abundance. However, bank voles predominated in the diet during laying early in the season (snow depth > 40 cm). Later in the season (snow depth ≤ 40 cm) the porportion of field voles and grey-sided voles increased during laying, suggesting increased availability of these species as the snow melted. Breeding density (percentage of nest boxes with ≥ 1 egg) showed a positive correlation whereas laying date showed a negative correlation with food supply in autumn. In contrast, clutch size, number of fledglings per successful nest, and an index of the annual production of fledglings showed positive correlations with food supply in spring. Clutch size declined with season but at different levels in different years, and laying date per se only explained 11% of clutch size variation among years. In contrast, the "year effect" (related to food supply) explained 29% of the variation in clutch size. Clutches were even larger at later laying dates (when, however, the food supply was better) in 1984 than in 1985. Nest survival and survival per egg (until fledging) in successful nests varied over the years, but did not covary with the cyclic food supply. However, the lowest nest survival was found in a year when voles declined dramatically over winter. Egg size did not show any variation among years.