Extrapair paternity and the opportunity for sexual selection in a socially monogamous passerine

Effects of alternate mating strategies on the opportunity for sexual selection are widely debated, and recent studies have concluded that the effects of extrapair (EP) paternity on the opportunity for sexual selection may have been overstated due to 1) methodological limitations of empirical studies and 2) the potential for males to gain from additional within-pair (WP) reproductive opportunities. We therefore examined the impact of EP paternity on the opportunity for sexual selection in the socially monogamous and single-brooded eastern kingbird (Tyrannus tyrannus). EP paternity was common in all 3 years of our study (61% of 89 broods, 47% of nestlings) and realized reproductive success (EP + WP young) ranged from 0 to 9 young/male/year. A total of 31% of males lost all WP paternity (24% sired neither WP nor EP young, whereas 7% sired EP but not WP young), and variance in male realized reproductive success was more than 9 times greater than that of apparent reproductive success. Nearly half of EP mates were not nearest neighbors, and many were separated by 3 or more territories (>1000 m). EP success was independent of nest defense behavior, but early singing males and males with high song rates were most successful at both a population level and when cuckolders and cuckoldees were compared. EP paternity contributed significantly to the opportunity for sexual selection in kingbirds, and we suggest that this is probably due to the low potential for WP variation in reproductive success, apparent long-distance movements of one or both sexes, and consequent absence of reciprocal cuckoldry.