Extrapair paternity and variance in reproductive success related to breeding density in Bullock's orioles

Abstract

Breeding density is predicted to influence the use of alternative reproductive strategies and may account for some of the variation in extrapair paternity rates found within and between avian species. By exploiting the extreme range of breeding densities occurring in a single, semicolonial population of Bullock's oriole, Icterus galbula bullockii, this study showed that, as predicted, both the number of nests containing extrapair nestlings and the mean proportion of extrapair nestlings in a brood increased with breeding density (high versus low breeding density: broods with extrapair nestlings: 62 versus 27%; mean: 48 versus 16%). As found in some other studies, extrapair paternity (both exclusion and assignment) significantly increased the variance in reproductive success among males. No difference was found in the mean reproductive success of males at different breeding densities. However, as predicted, the variance in reproductive success among males increased with breeding density. This study indicates that the distribution of individuals within a single population can lead to significant differences in the opportunity for sexual selection, and that density may be an important factor affecting variance in male reproductive success among populations.