Seasonal and Individual Variation in the Production of Offspring in the Ural Owl Strix uralensis

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Abstract
I studied a population of Ural owls in Southern Finland in 1977-88. I classified the breeding seasons into poor, intermediate and good according to winter food supply and winter severity. The number of pairs varied from twenty-nine to eight-five, and each year between 12% and 87% of them bred. In good years 75% of the pairs bred, in intermediate years 70% and in poor years 21%. Breeding females laid two to seven eggs per clutch. The mean clutch size varied from 2.22.+-.0.47 (S.D.) eggs in poor years through 3.22.+-.0.84 in intermediate years to 3.98.+-.1.15 in good years. The clutch size decreased by 0.07 eggs per day with advancing laying date. The model for the seasonal decline in clutch size explained 80% of the variation in mean yearly clutch sizes. For all clutches, 21.7% of eggs failed to hatch, and 4.3% of hatched young died in the nest. Brood size varied from 1.17.+-.1.07 to 3.21.+-.1.47, and 81% of the variation in brood size was explained by the variation in clutch size. Production of offspring per pair per year varied nearly sixteen-fold from 0.15.+-.0.51 to 2.38.+-.1.90. In poor years Ural owls produced 0.32 young per pair, in intermediate years 1.43 and in good years 2.11. Within clutch-size classes, the number of young did not vary between year-quality classes. Laying date had a significant between-female variance component of 20%, but the variance component for clutch size was only 1%. Fifty-one females, which were assumed to have started and ended their reproductive life during the study produced on average 7.7.+-.5.7 (range 0-18) young during their life (mean 4.5.+-.2.6, range 1-11 breeding seasons). The number of offspring increased by 2.01 for every year the females could persist in the breeding population. I suggest that, for long-lived species, it would be possible to estimate lifetime reproductive success by using females that are still alive and breeding.