FEMALE RED-WINGED BLACKBIRDS ACCRUE MATERIAL BENEFITS FROM COPULATINGWITH EXTRA-PAIR MALES

This study tested whether female red-winged blackbirds, Agelaius phoen iceus, acquire non-genetic, material benefits from copulating with mul tiple males. In this population, females solicit extra-pair copulation s, primarily from males defending adjacent territories on the same bre eding marsh. Results of a feeder experiment revealed that females that copulated with an extra-pair male were allowed to feed on his territo ry, but females that remained faithful to their social mates were acti vely excluded from foraging on neighbouring territories. Males discrim inated between faithful and unfaithful social mates. Females that enga ged in extra-pair copulations were prevented from foraging on their so cial mate's territory significantly more often than females that did n ot. Measurement of neighbouring male responses to a mounted nest preda tor, a black-billed magpie, Pica pica, revealed that males were also s ignificantly more aggressive towards a predator at a nest on a neighbo uring territory if they had copulated with the breeding female than if they had not. These findings provide a possible explanation for resul ts of Gray (1997, Anim. Behav., 53, 605-623), which showed that fledgi ng success was significantly higher for females that engaged in extra- pair copulations than for females that did not, because depredation ra tes were significantly lower in clutches laid by females that had enga ged in extra-pair copulations, and starvation rates tended to be lower in broods containing extra-pair young. This study is the first to dem onstrate that females can benefit materially as well as genetically by copulating outside their pair bond and illustrates the need for manip ulative tests to understand fully the reproductive strategies of indiv iduals in populations where social relationships often do not reflect genetic relationships. (C) 1997 The Association for the Study of Anima l Behaviour.