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-pairfertilizations can all affect male reproductive success in biparentalspecies. Extra-pair mating in birds has been of particular interest,because it could generate strong levels of sexual selectioneven when a species is socially monogamous. We examined howextra-pair fertilizations affect the opportunity for selectionin 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 accountingof male reproductive success. Although extra-pair fertilizationswere common in yellowthroats (26% of young) it contributed little(21%) to the total variance in male reproductive success. Mostof the variance in reproductive success (58%) was attributableto the male's within-pair success, which was influenced primarilyby the number of young produced by each mate and the proportionof within-pair young sired. Despite a moderate level of extra-pairfertilizations (10% of young) in house wrens, almost all ofthe variance in male reproductive success (97%) was attributableto within-pair success, particularly the number of social mates.Although extra-pair fertilizations generally increase the variancein male reproductive success, within-pair reproductive successmay be the major source of variation in male reproductive success.Thus, sexual dimorphism in monogamous birds may be influencedmore by the number of mates and their fecundity than by extra-pairmatings.