Effects of variable feeding conditions on the Tawny Owl Strix aluco near the northern limit of its range

Abstract

Relationships between the wintering conditions, vole abundance, and food and breeding success of a population of Tawny Owls in Uusimaa, southern Finland, were studied during a 15-year period (1986-2000). There were no significant trends in the weather variables studied, though general experience suggested that winters were getting milder. The abundance of small microtine voles, the staple food for owls before the breeding season, fluctuated in three-year cycles, but the general level of vole abundance seems to have lowered after the 1980s . Correspondingly, during the first years of the study the proportion of small voles in the prey of owls was significantly higher than later. When voles were scarce, the proportion of alternative prey in the diet of owls was high. The number of nestings (denoting the breeding frequency of owls) could be predicted from the abundance of voles in spring . On average, the Tawny Owls of the district laid eggs and produced young slightly but not significantly more in "good" vole years than in "poor" ones . The difference was, however, significant in the breeding success of the local population studied. The number of young fledged correlated positively with the number of voles brought to the nest by parents. Significant relationships between the clutch size and fledgling production of owls and the vole abundance and weather variables studied emerged only when the data of the "poor" vole years were excluded . In such cases there was a significant negative relationship between the average clutch size and the "frost see-saw effect", the number of days in winter during which the temperature at least once fell from plus to minus °C . The latter variable was suggested to be negatively correlated with the abundance of voles. The relationship was similar also with the average fledgling production that was, however, largely determined by the clutch size.