Male western bluebirds that sire extrapair young are also successful within-pair mates

Abstract

Extrapair (EP) paternity has the potential to enhance the reproductive success of males in socially monogamous systems, but increased success is not a given if seeking extrapair copulations is costly, causing males to lose paternity or experience reduced success of their own nests. Here we used 6 years of paternity data from a long-term study of western bluebirds, Sialia mexicana, to investigate the potential for reproductive consequences of EP paternity. We predicted that if females control paternity, and males gaining EP paternity are also attractive to their social mates and can thwart competition for fertilizations in their own nests, overall reproductive success should be increased compared to non-EP sires. In support of this prediction, we found that successful EP males sired almost twice as many young as males not gaining EP paternity; in addition to EP young, they had more within-pair (WP) young in their own nests, more successful nests and more fledglings per successful nest. Furthermore, through an analysis of the components of annual fitness, we found that variation in mate number contributed substantially to variation in total reproductive success. Together these data indicate that in western bluebirds, males gaining EP paternity are extremely successful and do not gain EP paternity at the expense of WP paternity. Finally, because successful EP sires were significantly older than males who were unsuccessful at acquiring EP paternity, this study also suggests that selection for longevity may be strong in this population.