Breeding synchrony and extrapair fertilizations in two populations of red-winged blackbirds

We tested the relationship between synchrony of breeding and the frequency of extrapair fertilizations (EPFs) in two populations of red-winged blackbi rds known to differ in female extrapair behavior. We found no association b etween the number of simultaneously fertilizable females (temporal neighbor s) and EPF rate in either population, although a significant difference bet ween populations in the direction of this relationship (positive where fema les initiated extrapair copulations and negative where males initiated them ) suggested a modest difference in the influence of synchrony. Males losing offspring to EPFs tended to have more fertilizable females at that time th an the actual sires in some analyses but not in others. We also tested seve ral assumptions underlying two competing hypotheses for the effects of sync hrony. We found no evidence that females pursued extrapair copulations more often when other females were synchronous. Rather, females were more likel y to gain EPFs with extrapair males whose social mates were not yet buildin g their nests. Synchrony also did not consistently affect male pursuit of e xtrapair copulations or achievement of EPFs. These results suggest that tim ing of breeding has some effects on extrapair activity, but that those effe cts are both relatively weak and influenced by other factors that vary betw een years or populations.