High opportunity for sexual selection in both sexes of an obligate brood parasitic bird, the brown-headed cowbird (Molothrus ater)

Abstract.

Obligate brood parasites provide no parental care for their offspring and as such offer an opportunity to investigate the relationship between mating behaviour and sexual selection in the absence of the constraints imposed by parental care. In brood parasites, the mating system is predicted to be polygamous and the opportunity for selection to act on each sex is expected to be high relative to species that provide care. We used microsatellite DNA-based parentage analyses to collect data on the individual reproductive and mating success of an obligate avian brood parasite, the brown-headed cowbird. We determined the levels of multiple mating in each sex, the relationship between the reproductive success and mating success (sexual selection gradients) and quantified the opportunities for sexual selection to act on males and females (standardized variance in mating success). We found that: (1) males (annual mean 58%±26% SD) and females (annual mean 31%±19%) produced offspring with more than one individual of the opposite sex; (2) there was a significant positive sexual selection gradient of similar magnitude for both sexes; and (3) the opportunity for selection was high in both sexes (male = 1.52; female = 0.73). These results demonstrate that cowbirds have a polygamous genetic mating system and that annual reproductive success was positively associated with annual mating success for both sexes. As predicted, the opportunity for selection was relatively higher for a brood parasitic species compared to non-parasitic species for which data were available.