REFLECTIONS OF WINTER SEASON LARGESCALE CLIMATIC PHENOMENA AND LOCAL WEATHER CONDITIONS IN ABUNDANCE AND BREEDING FREQUENCY OF VOLE-EATING BIRDS OF PREY
Authors: Tapio Solonen and Pertti Saurola
Abstract: We examined long-term (1986–2008) data on the number of occupied territories and breeding frequency (active nests) of nine species of vole-eating birds of prey in southernmost Finland, using generalized linear models. Explaining variables included wintertime and monthly large-scale climatic conditions indicated by North Atlantic Oscillation (NAO), mean winter and monthly mean ambient temperature and depth of snow cover at five local weather stations, as well as indices of autumn and spring abundance of voles at three localities within or near to the study area. The birds of prey included six site-tenacious species, of which four (Bubo bubo, Glaucidium passerinum, Strix aluco, Strix uralensis) were mainly sedentary and two (Circus aeruginosus, Buteo buteo)migratory ones, and three more or less nomadic species (Falco tinnunculus, Asio otus, Aegolius funereus). We expected that climatic effects were expressed in the numbers and breeding performance of birds of prey largely via their effects on highly fluctuating vole populations. In accordance with earlier findings, numbers and breeding of vole-eaters were largely governed by the abundance of small voles, confirming the suitability of our data to the present purpose. Large-scale climatic phenomena, indicating mild winter conditions, presented a nearly significant positive influence on the numbers and breeding frequency of southerly distributed site-tenacious species (Buteo buteo, Bubo bubo, Strix aluco). The combined effect of vole abundance and local mean winter temperature was negative both in sedentary Strix aluco and nomadic Falco tinnunculus. High temperatures in the beginning and at the end of winter showed positive associations. Thick snow cover combined with vole abundance showed positive associations with numbers and breeding frequency of various kinds of vole-eating birds of prey. The results followed largely our expectations though the link via vole abundance was inadequately demonstrated. Our results suggest that the effects of global warming on various vole-eating birds of prey at high latitudes were both positive and negative, in particular due to mild winters. This would lead to changes in local populations and distribution ranges of species. Due to their flexible moving habits, nomadic species might be less seriously affected than site-tenacious ones that are more dependent on local resources, such as nest sites. From a local point of view and during a short period of time, however, the impact seemed to be more pronounced on nomadic species due to their sudden and drastic shifts.