Do female red-winged blackbirds benefit genetically from seeking extra-pair copulations?

Female pursuit of extra-pair copulations was investigated in a western population of red-winged blackbirds, Agelaius phoeniceus. Potential genetic benefits to females were considered in this study; potential material gains were examined in Gray (1997; Anim. Behav., 53, 625-639). In this population, females initiate courtship and copulation outside their social bond, and between 18 and 43% of all breeding females engaged in extra-pair copulations each breeding season. The peak of extra-pair copulation occurred 2 days before the first egg was laid and I day closer to egg laying compared to the peak of within-pair copulation, suggesting that extra-pair copulations conferred a genetic advantage to females. Females that copulated with extra-pair males improved their reproductive success by hatching a significantly greater proportion of eggs and hedging a significantly greater proportion of young than females that did not copulate with extra-pair males. Improved hatching success was probably due to female avoidance of temporary male sperm depletion. Improved hedging success resulted from significantly lower depredation rates of clutches laid by females that copulated with multiple males. In addition, a lower proportion of nestlings tended to starve in broods containing extra-pair fertilized offspring. I found no evidence that participation in extra-pair copulations was costly to females. Male aggression towards females was rare, and males were never observed chasing or force-copulating with a mate immediately following an extra-pair copulation. These results show that the selection pressure on females to engage in extra-pair copulations is strong, and they support new theories maintaining that extra-pair copulations are an important aspect of female reproductive success. (C) 1997 The Association for the Study of Animal Behaviour.