Effects of extra-pair and within-pair reproductive success on the opportunity for selection in birds

The number of mates, their fecundity, and the number of extra-pair fertilizations can all affect male reproductive success in biparental species. Extra-pair mating in birds has been of particular interest, because it could generate strong levels of sexual selection even when a species is socially monogamous. We examined how extra-pair fertilizations affect the opportunity for selection in the sexually dimorphic common yellowthroat (Geothlypis trichas) and the sexually monomorphic house wren (Troglodytes aedon). We were able to identify sires for at least 95% of all nestlings, and, thus, we were able to make a nearly complete accounting of male reproductive success. Although extra-pair fertilizations were common in yellowthroats (26% of young) it contributed little (21%) to the total variance in male reproductive success. Most of the variance in reproductive success (58%) was attributable to the male's within-pair success, which was influenced primarily by the number of young produced by each mate and the proportion of within-pair young sired. Despite a moderate level of extra-pair fertilizations (10% of young) in house wrens, almost all of the variance in male reproductive success (97%) was attributable to within-pair success, particularly the number of social mates. Although extra-pair fertilizations generally increase the variance in male reproductive success, within-pair reproductive success may be the major source of variation in male reproductive success. Thus, sexual dimorphism in monogamous birds may be influenced more by the number of mates and their fecundity than by extra-pair matings.