Seasonal changes in diet of Hawk Owls Surnia ulula: importance of snow cover

Abstract

The diet of three pairs of the Hawk Owl Surnia ulula nesting in the boreal zone of SE Norway was followed by analysing pellets collected at average intervals of six days during the breeding season, and compared with the occurrence of vole prey in the two main coniferous forest habitats of the species. When the ground was completely snowcovered, the smaller Bank Voles Clethrionomys glareolus, found in only mature forest, predominated in the diet, but when the ground was partly or completely snow-free, the larger Field Voles Microtus agrestis and Root VolesM. oeconomus, found only in clearcuts, were the dominant prey. This change occurred rapidly, and coincided with the first appearance of snow-free ground. The data were consistent with the predictions of changes in diet based on optimal foraging theory when prey density was replaced by prey availability, as influenced by snow cover. Snow mayinfluence the population dynamics of Bank Voles and Field/Root Voles differently by causing differentdegrees ofexposure to specialist avian predators.