Clutch Size, Breeding Success and Brood Size Experiments in Tengmalm's Owl Aegolius funereus: A Test of Hypotheses

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Abstract
During 1973-85, the mean clutch size and the mean number of fledglings in Tengmalm''s Owl correlated positively with the spring trap index of the preferred prey (Microtus voles). The largest clutches (7-10 eggs) were more profitable both per egg and per clutch than the most common clutches (5-6) which, in turn, were more productive than the smallest clutches (1-4). Predation rate was higher on small clutches than on large ones. Broods with one young experimentally added did not suffer from a higher nestling mortality than control broods or broods with one young experimentally reduced. This suggests that the amount of food available to the egg-laying female is the critical factor for the determination of clutch size. However, young of enlarged broods tended to achieve slightly lower weight at fledging than did young from reduced broods, which indicates that their phenotypic quality and therefore later survival might be lower. This may select for smaller clutches than those producing the largest number of fledglings.