Is the long-term decline of boreal owls in Sweden caused by avoidance of old boxes?

Abstract

Numbers of Boreal Owls (Aegolius funereus) breeding in nest boxes in northern Sweden have declined since the 1980s. The main cause of this decline is thought to be a similar decline in voles, the owls' main prey. However, an alternative reason for the decline might be the aging of nest boxes and the owls' avoidance of old nest boxes to reduce the risk of predation. In this study we tested this alternative hypothesis in an experiment during the first two years of a 3-yr vole cycle by comparing predation and the number of breeding owls in old and new nest boxes at their original location, and old and new nest boxes placed at new locations. Predation was lower at relocated nest boxes, but Boreal Owls showed no preference for any of the four nest-box treatments, and breeding parameters did not vary between treatments. We conclude that the decline in the number of Boreal Owls breeding in nest boxes is real, and not caused by aging of nest boxes. © 2013 The Raptor Research Foundation, Inc.