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I examined the effect of harem size on female reproductive success in the red - winged blackbird (Agelaius phoeniceus) in Ontario while controlling for the confounding effects of territory and male quality. Male territories were matched by their quality and harem sizes were manipulated by selective removal of females. The removals created monogamous and bigamous harems. Bigamous females fledged significantly fewer young than monogamous females, mainly due to higher predation on their nests. Young of the bigamous females were fed less frequently than young of the monogamous females (mainly due to reduced male assistance), but the difference was not significant. There was a positive relationship between parental provisioning rate and nestling body size. Young of the bigamous females fledged at smaller body size than young of the monogamous females. Because fledgling body size is related to post - fledgling survival, young of the bigamous females presumably experienced lower survival than young of the monogamous females. The lower number of fledglings, combined with their lower survival, suggests that the bigamous females produced fewer descendants than the monogamous females. I conclude that polygyny is costly to females in this population of the red - winged blackbird.

Previous experimental studies demonstrated that females in this population prefer to settle with unmated males rather than already - mated males. In light of the present findings, the preferences appear adaptive, as they reduce the cost of polygyny. In Pennsylvania, Searcy (1988) reported that females settle independently of harem size and that harem size has no effect on their reproductive success. A comparison of the Ontario and Pennsylvania populations suggest that there are geographic differences in the effect of harem size on both female preferences and reproductive success.

Affiliations: 1: Department of Biology, University of Miami, Coral Gables, FL 33124-0421, USA


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