Birds that winter in cold northern climates experience harsh conditions, including reduced food availability and increased energy demands. In raptors, the ability to forage and maintain body condition may be related to age (hunting experience) or the ability to defend good-quality territories (dominance). We examined the effects of age and sex on body condition and various sources of mortality in wintering Snowy Owls (Bubo scandiacus) on the Canadian prairies. Because of reversed sexual size dimorphism, we predicted that females, the dominant sex, would be in better condition than males and that adults would be in better condition than juveniles. Consistent with these predictions, data from 537 live Snowy Owls trapped over 18 winter field seasons showed that adults were heavier than juveniles for a given body size and carried more fat reserves. We found that 56% of males lacked furcular and wing-pit fat, whereas only 31% of females lacked such fat; and females, but not males, tended to put on fat during the winter months. A comparison of the sex ratio of starving Snowy Owls turned in to rehabilitation centers (63% male) and that of living Snowy Owls observed in the wild (45% male) showed a male bias in starving and diseased individuals. Although most of the wild-trapped birds were above the starvation threshold, the proximate mechanisms by which sex-biased competitive dominance is manifested in differences in body condition and survival warrant further study.
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Vol. 133 • No. 4