Effects of snow cover and rain on breeding success in tawny owls Strix aluco

Abstract: Breeding success in Tawny owls is known to be affected by both prey abundances and weather conditions. In Finland, an effect of winter conditions on hatching date was found and in Hungary it has been shown that snow cover affect chick survival negatively. In this study the effect of snow cover on clutch size and egg and chick survival is investigated on a population of Tawny owls in Västra Götaland, Sweden. Since rainfall has been shown to have a disturbing effect on incubation and breeding, the amount of rainfall during breeding season was compared to egg and chick survival. No significant effect on clutch size was found due to snow cover. On egg and chick survival, snow cover had a significant negative effect when other important factors, e.g. access to prey and the amount of inexperienced and young breeders, was controlled. A significant positive correlation was found between rainfall and egg and chick survival, but no significant effect on rainfall was found after controlling for other factors such as the amount of caught prey. There was also a significant correlation between rainfall and the percentage of birds in prey, where years with more rainfall had less birds in prey, a variable that usually is higher during low vole abundances. To support themethod of using prey found in nest boxes as indicators of prey abundances it was shown that the total amount of prey found in nest boxes and the percentage of birds in prey had a positive respectively negative significant effect on chick survival. These results indicate that the chances of chick survival might be affected before the eggs are laid and that there is an indirect positive effect of rain on breeding success due to higher amounts of preferred prey.