Effects of Variable Food Supply on the Body Condition of Breeding Tawny OwlsStrix alucoin Southern Finland

Body condition of boreal species of vole-eaters seems to vary in accordance with fluctuating vole populations. Tawny owls Strix aluco of temperate origin were studied to reveal if they show a similar pattern during nestling period near the northern limit of the species' range. The body condition of tawny owls was not considerably affected by the annual fluctuations of small voles indicated by regional vole indices and local prey samples. There were some significant relationships with some alternative prey but the variation in the body condition of owls was largely explained by the bird's age, time of the season, and stage of the nestling period. Old birds that bred earlier in the season were in better condition than young ones that bred later. Due to alternative prey, the body condition seemed to be relatively resistant to the effects of variable foraging conditions as expected for a generalist species of southern origin.