Nest-Defense Behavior in the Red-Winged Blackbird

Abstract

We examined various aspects of nest-defense behavior in marsh-nesting Redwinged Blackbirds (Agelaius phoeniceus) in Madison, Wisconsin. Blackbirds used seven types of calls in nest defense. The female scream elicited the strongest response of any call type; attracting more birds, and eliciting the highest rate of hovers. Additionally, the scream caused begging nestlings to stop begging and crouch low in the nest. Female blackbirds had higher call rates to predator models than did male blackbirds; however, males were the more aggressive sex in defense of the nest. Although there were no differences in call rates to different predator models, there were differences in more overt nest-defense behavior. The ability to drive off a predator best explained the variation in aggressiveness to the predator models. Male blackbirds, when faced with a choice, defended nests of primary females more aggressively than nests of secondary females; the differences were due to neither the nests' contents nor to the stage of the nests. Call rates, but not rates of dives and strikes, were correlated with successful nests. Finally, the most aggressive males had the largest harems.