Microsatellite identification of extrapair sires in a socially monogamous warbler

Few studies of avian mating systems have identified the sires of extrapair young, and hence it has been difficult to determine the scale at which repr oductive interactions occur. For instance, females may be free to copulate with any male in the population (a "global" scale of interactions), or fema les may be restricted to copulating only with males on neighboring territor ies (a "local" scale). The scale of such interactions has important consequ ences for an understanding of the evolutionary causes and consequences of e xtrapair fertilizations. We used five hypervariable microsatellite loci and multilocus DNA fingerprinting to examine parentage of more than 400 nestli ng black-throated blue warblers (Dendroica caerulescens). Extrapair fertili zations were common. and the microsatellite markers allowed us to identify the sires for 89% of the young analyzed. Most identified extrapair sires we re males on neighboring or nearby territories, and most nestlings for whom we could not identify a sire came from territories at the edge of the study plot. Thus, reproductive interactions appear to he more local than global in this population. Extrapair fertilizations contributed significantly to t otal variation in male reproductive success. However, the standardized vari ance in male reproductive success (0.68-0.74) was not substantially greater than that for females (0.53-0.60), and the contribution of extrapair ferti lizations 19-14%) was much lower than the contribution of within-pair ferti lizations (75-77%). This suggests that the local scale of reproductive inte ractions may limit variation in male reproductive success and hence the opp ortunity for selection.