Experimental evidence for the cost of polygyny in the red-winged blackbirdAgelaius phoeniceus

It 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 we re matched by their quality and harem sizes were manipulated by selective r emoval of females. The removals created monogamous and bigamous harems. Big amous females fledged significantly fewer young than monogamous females, ma inly due to higher predation on their nests. Young of the bigamous females were fed less frequently than young of the monogamous females (mainly due t o reduced male assistance), but the difference was not significant. There w as 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 experienc ed 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 bl ackbird. Previous experimental studies demonstrated that females in this population prefer to settle with unmated males rather than already-mated males. In lig ht 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 th eir reproductive success. A comparison of the Ontario and Pennsylvania popu lations suggest that there are geographic differences in the effect of hare m size on both female preferences and reproductive success.