Female red-winged blackbirds accrue material benefits from copulating with extra-pair males

This study tested whether female red-winged blackbirds,Agelaius phoeniceusacquire non-genetic, material benefits from copulating with multiple males. In this population, females solicit extra-pair copulations, primarily from males defending adjacent territories on the same breeding marsh. Results of a feeder experiment revealed that females that copulated with an extra-pair male were allowed to feed on his territory, but females that remained faithful to their social mates were actively excluded from foraging on neighbouring territories. Males discriminated between faithful and unfaithful social mates. Females that engaged in extra-pair copulations were prevented from foraging on their social mate's territory significantly more often than females that did not. Measurement of neighbouring male responses to a mounted nest predator, a black-billed magpie,Pica picarevealed that males were also significantly more aggressive towards a predator at a nest on a neighbouring territory if they had copulated with the breeding female than if they had not. These findings provide a possible explanation for results of Gray (1997,Anim. Behav.53, 605–623), which showed that fledging success was significantly higher for females that engaged in extra-pair copulations than for females that did not, because depredation rates were significantly lower in clutches laid by females that had engaged in extra-pair copulations, and starvation rates tended to be lower in broods containing extra-pair young. This study is the first to demonstrate that females can benefit materially as well as genetically by copulating outside their pair bond and illustrates the need for manipulative tests to understand fully the reproductive strategies of individuals in populations where social relationships often do not reflect genetic relationships.