We tested the relationship between synchrony of breeding and the frequency of extrapair fertilizations (EPFs) in two populations of red-winged blackbirds known to differ in female extrapair behavior. We found no association between the number of simultaneously fertilizable females (temporal neighbors) and EPF rate in either population, although a significant difference between populations in the direction of this relationship (positive where females 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 than the actual sires in some analyses but not in others. We also tested several assumptions underlying two competing hypotheses for the effects of synchrony. We found no evidence that females pursued extrapair copulations more often when other females were synchronous. Rather, females were more likely to gain EPFs with extrapair males whose social mates were not yet building their nests. Synchrony also did not consistently affect male pursuit of extrapair copulations or achievement of EPFs. These results suggest that timing of breeding has some effects on extrapair activity, but that those effects are both relatively weak and influenced by other factors that vary between years or populations.