PATTERNS AND CORRELATES OF EXTRAPAIR PATERNITY IN AMERICAN REDSTARTS (SETOPHAGA-RUTICILLA)

We examined correlates and hypotheses pertaining to extrapair fertiliz ations 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 distribut ion of extrapair young in the population. This result is consistent dt h the ''good genes'' hypothesis, but not with the ''genetic diversity' ' hypothesis. There was a negative association between the age of puta tive fathers and the proportion of extrapair young in their broods. Ir respective of age, males with prior residency were cuckolded less ofte n than males new to the study area. Extrapair fathers were immediate n eighbors in 7 of 10 cuckolded broods where all neighbors were sampled, Males were more likely to sire offspring in the territories of younge r 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 reproductiv e 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 ma les whose own nests were successful also had extrapair young. There wa s no egg dumping by females. We conclude that male age and prior resid ency were predictors of cuckoldry in American redstarts. In the contex t of the heavy predation experienced by our birds, extrapair fertiliza tions allowed many males to salvage some reproductive success and did not increase the variance of success across males.