Patterns and correlates of extrapair paternity in American redstarts (Setophaga ruticilla)

We examined correlates and hypotheses pertaining to extrapair fertilizations in socially monogamous American redstarts (Setophaga ruticilla). DNA fingerprinting revealed extrapair fertilization in 59% of broods (19 of 32), involving 40% of nestlings (43 of 108). Fewer broods than expected had mixed paternity, as determined from a binomial distribution of extrapair young in the population. This result is consistent dth the "good genes" hypothesis, but not with the "genetic diversity" hypothesis. There was a negative association between the age of putative fathers and the proportion of extrapair young in their broods. Irrespective of age, males with prior residency were cuckolded less often than males new to the study area. Extrapair fathers were immediate neighbors in 7 of 10 cuckolded broods where all neighbors were sampled, Males were more likely to sire offspring in the territories of younger neighbors than in those of older neighbors. Plumage characteristics of adult males, breeding synchrony of females, and breeding densities were not significantly associated with cuckoldry. Realized reproductive gain from cuckoldry was small because of high nest predation in our area. Extrapair fertilizations allowed one-quarter of males whose own nests had failed to achieve same reproductive success. Only 2 of 17 males whose own nests were successful also had extrapair young. There was no egg dumping by females. We conclude that male age and prior residency were predictors of cuckoldry in American redstarts. In the context of the heavy predation experienced by our birds, extrapair fertilizations allowed many males to salvage some reproductive success and did not increase the variance of success across males.