Extrapair paternity and local synchrony in the Black-throated Blue Warbler

We examined genetic parentage in a population of Black-throated Blue Warble rs (Dendroica caerulescens) in New Hampshire during 1995 and 1996. Although parentage patterns have not been examined in this socially monogamous spec ies, behavioral observations have suggested that extrapair fertilizations ( EPFs) occur. Using multilocus DNA fingerprinting, we determined parentage o f 125 offspring in 38 broods. EPFs were present in 17 broods (44.7%) and in cluded 34 offspring (27.2%) over both years. We found no evidence of intras pecific brood parasitism, because all offspring had high band-sharing score s with their social mothers. Frequencies of EPFs did not differ significant ly between years (although only two years were examined). There was also no significant effect of time of season, age of social parents, population le vels of breeding synchrony, or breeding density upon frequency or presence of EPFs within broods. In contrast, the occurrence of EPFs was strongly and positively associated with breeding synchrony among neighboring females; n ests in areas of high local synchrony were more likely to contain extrapair young. Univariate logistic regression indicated that the level of local sy nchrony accounted for 22% of the variance in probability of EPF. These resu lts suggest that the temporal and spatial concentration of fertile females are important in influencing the prevalence of EPFs.