Extrapair Paternity and Local Synchrony in the Black-Throated Blue Warbler

Abstract

We examined genetic parentage in a population of Black-throated Blue War- biers (Dendroica caerulescens) in New Hampshire during 1995 and 1996. Although parentage patterns have not been examined in this socially monogamous species, behavioral obser- vations have suggested that extrapair fertilizations (EPFs) occur. Using multilocus DNA fin- gerprinting, we determined parentage of 125 offspring in 38 broods. EPFs were present in 17 broods (44.7%) and included 34 offspring (27.2%) over both years. We found no evidence of intraspecific brood parasitism, because all offspring had high band-sharing scores with their social mothers. Frequencies of EPFs did not differ significantly between years (although only two years were examined). There was also no significant effect of time of season, age of social parents, population levels of breeding synchrony, or breeding density upon fre- quency or presence of EPFs within broods. In contrast, the occurrence of EPFs was strongly and positively associated with breeding synchrony among neighboring females; nests in ar- eas of high local synchrony were more likely to contain extrapair young. Univariate logistic regression indicated that the level of local synchrony accounted for 22% of the variance in probability of EPE These results suggest that the temporal and spatial concentration of fertile females are important in influencing the prevalence of EPFs. Received 4 May 1998, accepted 4 January 1999. MATE FIDELITY is not universal in socially monogamous breeding systems, as demon- strated by extrapair fertilizations (EPFs) that result when a female copulates with a male with whom she is not socially paired. In recent years, many genetic studies of passerines have shown EPFs to be prevalent in some species but absent in others (Westneat and Webster 1994, Webster and Westneat 1998). High variability in EPF rates across species creates the need for an assessment of the factors affecting EPF rates within species (Petrie and Kempenaers 1998). Breeding density is thought to be one factor contributing to variation in EPF rates among species. High breeding density potentially in- creases interactions between the sexes, which may in turn influence mating behaviors and re- suit in an increase in EPFs (Westneat and Sher- man 1997). However, considering density alone neglects the status of females with respect to their ability to be fertilized; proximity of indi- viduals should be examined in conjunction with the fertility status of females (i.e. breeding synchrony) in the area. Breeding synchrony at the population level has been examined sepa-