The goal of this study was to evaluate the relative role of some central intrinsic and extrinsic factors affecting reproduction in the tawny owl in southern Finland, near the northern limit of the species' range. The data included 153 successful nestings of pairs of birds in which both mates were identified, sexed, aged, and measured. Brood size was constrained by clutch size that, in turn, could be explained by the positive influence of female age. Female condition was positively influenced by both male condition and female age, while male condition was positively related to the age of male. These intrinsic factors outweighed the effects of extrinsic factors examined. However, these probably prominent extrinsic factors, such as general food supply and winter weather conditions, seemed to be inadequately quantified for use in the present study. Therefore, future studies should proceed by evaluating variables that characterize more accurately those environmental conditions where the studied population lives.
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Vol. 46 • No. 4