Reversed Size Dimorphism in Birds of Prey, Especially in Tengmalm's Owl Aegolius funereus: A Test of the "Starvation Hypothesis"

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Abstract
Suggestions and predictions of hypotheses (especially those of the "starvation hypothesis") concerning reversed sexual size dimorphism of birds of prey were tested using field data collected in western Finland. Early breeding species in two communities of birds of prey were more dimorphic in weight than late-breeding ones. The degree of weight dimorphism between mates in 20 pairs of Tengmalm''s Owl correlated negatively with the timing of egg-laying, while the wing length of males correlated positively with the laying date of their partners. Clutch size and number of young produced decreased with degree of weight dimorphism within owl pairs. Data from Finnish owls did not support the hypothesis, that the need to defend the nest has caused the evolution of larger females, but partly agreed with the "starvation hypothesis", which states that selection acts on owl females to withstand food shortage in the breeding season. It seems that female Tengmalm''s Owls chose light, short-winged males because of their aerial agility; these males probably competed more efficiently for good territories and nest-holes and also hunted more effectively and economically. Because of their aerial agility, owl males tended to catch more birds and less mammals than did females. This niche separation may reduce intersexual competition for food and select for the high degree of size dimorphism.