Estimating components of variance in demographic parameters of Tawny Owls, Strix aluco

Survival rates of Tawny Owls (Strix aluco) were estimated using recapture and recovery data from approximately 20,000 nestling and adult owls ringed between 1980 and 1999 in southern Finland. Survival rates averaged 33% in the first year of life, 64% in the second, and 73% in subsequent years, but varied dramatically among years. Approximately 50% of annual variation in survival could be explained by stage of the vole cycle and severity of winter weather. Capture probabilities, an index of breeding propensity, varied dramatically among years, and could almost entirely be explained by the vole cycle, superimposed on a long–term increase in capture effort. Matrix models based on mean values in each year of the vole cycle, predict that in 2 out of 3 years, the population would decline by 13%–15% per year, offset by a large increase in the 3rd year. Numbers of nesting pairs are predicted to be low in one of three years, with no long–term trend, consistent with observed estimates of active nests.