Are vole-eating owls affected by mild winters in southern Finland?

Abstract

Relationships between winter temperatures, vole abundance, and the occurrence and breeding of various vole-eating owls were studied on the basis of Finnish data collected in 1986-2000. The species included two resident generalists (Tawny Owl Strix aluco and Ural Owl S. uralensis) and two nomadic vole specialists (Long-eared Owl Asio otus and Tengmalm's Owl Aegolius funereus). Near the southern coast, spring vole abundance contributed significantly to the occurrence and breeding of owls, while in inland populations they seem already to be largely governed by the vole abundance of the preceding autumn. The mild winter temperatures contributed negatively to the abundance of vole specialists, whereas Tawny Owls of southern distribution seemed to be both positively and negatively affected. When only the good vole years were included in the analyses, the positive effect of high winter temperatures on the occurrence of resident species and their negative effect on the clutch size and breeding success of owls, in general, were emphasised. When the mildness of winters near the southern coast of Finland was characterised by the number of the days during which the ambient temperature at least once fell from plus to minus °C (the intensity of the "frost seesaw"), additional significant relationships between mild winters and the occurrence and breeding of owls emerged. The frost seesaw contributed negatively, in combination with the positive contribution of vole abundance, to the occurrence and breeding frequency of nomadic vole specialists, and explained alone a considerable proportion of the variation in the breeding effort of all the species. In mild winters a lesser proportion of voles seemed to survive from autumn to spring, providing an explanation for the negative effects of mild winters on owls.