Extra-pair Paternity in the Dusky Warbler, Phylloscopus fuscatus: a Test of the 'Constrained Female Hypothesis'

Abstract

The 'constrained-female hypothesis' (CFH) states that females risk losing their social partner's assistance in brood care when they engage in extra-pair copulations. Hence, it is predicted that individual differences in female extra-pair behaviour can be explained in terms of (1) the amount of male care that females risk to lose (2) the intrinsic ability of females to raise offspring without male help, and (3) food availability, facilitating uni-parental care. The dusky warbler, a species with a polygynous mating system and a high rate of extra-pair paternity (45% of the offspring) offers an excellent opportunity to test all these predictions. In accordance with prediction (1) I find that females in pairing situations in which they cannot expect to receive male assistance in brood care are more likely to have extra-pair young than females in situations where bi-parental care is the rule. Using age and bill morphology as predictors for a female's ability to raise nestlings alone, I find no direct support for prediction (2). Finally, variation in food availability (3) had no influence on extra-pair paternity. Overall, the CFH provides some explanation for why female promiscuity differs between situations of bi-parental and uni-parental care, but the large amount of remaining variation can neither be explained by food abundance nor by individual female characteristics.