THE EVOLUTION OF PLUMAGE BRIGHTNESS IN BIRDS IS RELATED TO EXTRAPAIR PATERNITY

A positive association between plumage brightness of male birds and th e degree of polygyny may be the result of sexual selection. Although m ost birds have a socially monogamous mating system, recent paternity a nalyses show that many offspring are fathered by nonmates. Extrapair p aternity arises from extrapair copulations which are frequently initia ted by females. Not all females will be able to mate with a male of th e preferred phenotype, because of the mating decisions of earlier pair ed females; extrapair copulations may be a means for females to adjust their precopulation mate choice. We use two comparative analyses (sta ndardized linear contrasts and pairwise comparisons between closely re lated taxa) to test the idea that male plumage brightness is related t o extrapair paternity. Brightness of male plumage and sexual dimorphis m in brightness were positively associated with high levels of extrapa ir paternity, even when potentially confounding variables were control led statistically. This association between male brightness and extrap air paternity was considerably stronger than the association between m ale brightness and the degree of polygyny. Cuckoldry thus forms an imp ortant component of sexual selection in birds.