Microsatellite identification of extrapair sires in a socially monogamous warbler

Few studies of avian mating systems have identified the siresof extrapair young, and hence it has been difficult to determinethe scale at which reproductive interactions occur. For instance,females may be free to copulate with any male in the population(a "global" scale of interactions), or females may be restrictedto copulating only with males on neighboring territories (a"local" scale). The scale of such interactions has importantconsequences for an understanding of the evolutionary causesand consequences of extrapair fertilizations. We used five hypervariable microsatellite loci and multilocus DNA fingerprintingto examine parentage of more than 400 nestling black-throatedblue warblers (Dendroica caerulescens). Extrapair fertilizationswere common, and the microsatellite markers allowed us to identifythe sires for 89% of the young analyzed. Most identified extrapairsires were males on neighboring or nearby territories, andmost nestlings for whom we could not identify a sire came fromterritories at the edge of the study plot. Thus, reproductive interactions appear to be more local than global in this population.Extrapair fertilizations contributed significantly to totalvariation in male reproductive success. However, the standardizedvariance in male reproductive success (0.68-0.74) was not substantiallygreater than that for females (0.53-0.60), and the contributionof extrapair fertilizations (9-14%) was much lower than thecontribution of within-pair fertilizations (75-77%). This suggeststhat the local scale of reproductive interactions may limitvariation in male reproductive success and hence the opportunityfor selection.     

The effectiveness of mate guarding by male black-throated blue warblers

In many socially monogamous birds, males maintain close proximityto their mates during the fertile period. This is often consideredan effort on the male's part to prevent other males from copulatingwith his mate, but other functions have been suggested andthe effectiveness of males in preventing extrapair fertilizationshas come into question. Moreover, it is unclear whether mateguarding conflicts with other male activities, particularlythe pursuit of extrapair fertilizations. We examined mate guardingby male black-throated blue warblers (Dendroica caerulescens).Behavioral observations showed that males that guarded theirmates more closely were less likely to have extrapair youngin their nests. Moreover, the experimental detention of a malefor 1 h during the fertility risk period increased the probabilitythat a brood would contain extrapair young. Thus, male mate guarding was effective in reducing the risk of extrapair fertilization.Males with many opportunities for extrapair copulations appearedto guard their mates less and consequently had more extrapairyoung in their broods than males with few such opportunities.This suggests that mate guarding may conflict with the pursuitof extrapair fertilizations.