RESUMO
We examined opposing selective forces on female body size in the sexually dimorphic red-winged blackbird: social competition favoring larger females, and energetic advantages favoring smaller females. Downhower proposed that selection might drive female birds to be smaller than the optimum for survival, if smaller females were able to exceed their energetic requirements for self-maintenance earlier in the season and therefore breed earlier. Since in most birds the earliest breeders fledge the most young, this could favor the evolution of smaller female size, and therefore contribute to the magnitude of sexual size dimorphism in these birds. We tested this hypothesis in 1987 and 1988 by comparing the size and breeding date of female red-winged blackbirds. Consistent with our preditions, early-nesting females had much higher nesting success, but contrary to prediction, larger females bred earlier. We then examined the effects of female size on competition. If large females have an advantage in social competition, and if competition influences breeding date and reproductive success, then larger females might breed earlier. Primary females, the first females to arrive and nest on a territory, were more aggressive than lower ranked females; more aggressive females settled on better territories and laid earlier than less aggressive females; and larger females were more aggressive. Social competition between females may therefore favor large females. Finally, we tested the prediction that selection favoring large females might be limited by energetic constraints on large females. We found that large females had less fat than small females during breeding, and that the levels of fat that females of a given size carried affected breeding date and egg size. Therefore, social competition may favor large females, but reproductive energetics favoring smaller females may constrain selection for large female body size.