Female Body Condition and Breeding of the Ural Owl Strix uralensis

Female body condition affects reproductive success in birds of prey. Females with ample body reserves lay early and large clutches. Body reserves also act as a buffer against short-term food shortage. We analysed variation in the body condition of female Ural owls Strix uralensis in 1978-1991. Depending on the year, 80- 100 pairs occupied 160 nest-boxes in an area of 1500 km 2 in southern Finland. The weights of 199 females were converted into condition indices by calculating the difference between the observed body weight and the expected minimum weight, which was calculated from the regression equation between observed (at least three observations from different years) minimum weight and arm (ulna + radius) length. Female body condition depended on food supply and varied in accordance with the vole populations