Reproductive success and parental effort of Tengmalm's owls: Effects of spatial and temporal variation in habitat quality

Spatial and temporal variation in habitat quality may affect reproductive success and survival of animals. We examined habitat quality of vole-eating Tengmalm's owls Aegolius funereus for five years (1989-1992 and 1994) under cyclic food conditions in western Finland. We estimated food abundance (the most important component of habitat quality in Tengmalm's owl) by snap-trapping small mammals on individual territories and by censusing birds (an important alternative prey of owls). We classified territories according to the owl breeding frequency in an earlier 10-year period (mean: 2 breeding attempts per territory) as poor (0-2 breeding attempts) or good ones (> 3 breeding attempts). More Microtus and Clethrionomys voles were snap-trapped on good territories than on poor ones in the increase and peak vole years, but not in the low vole years. Fledgling production, but not egg production, increased with the number of voles snap-trapped on individual territories. The abundance of small birds was higher on good than on poor territories. In the low vole year, more small birds were censused near occupied than unoccupied nest-boxes of owls, which suggests that in poor vole years owls may shift to sites with abundant alternative prey within their territories. In the peak vole year, the size of food caches, food provisioning rate of males and nest defence intensity of females were higher on good than on poor territories. These results suggest that main and alternative food sources may have different importance over time in determining habitat quality and that the abundance of different prey types may affect parental effort and reproductive success of owls. Therefore, studies on species subsisting on cyclically fluctuating food sources should incorporate both temporal and spatial variation in resource abundance.