BREEDING SYNCHRONY AND EXTRA-PAIR MATING IN RED-WINGED BLACKBIRDS

Using data from a 6-year paternity study of red-winged blackbirds, I t ested the hypotheses that increased nesting synchrony should either pr omote extrapair mating by increasing the advantage of extra-pair matin g to females, or decrease extra-pair mating by constraining males from seeking extra-pair copulations. Contrary to these hypotheses, the occ urrence of extrapair paternity did not vary with nesting synchrony ove r the breeding season, or vary with the number of synchronous nests wi thin territories or within marshes, or with nesting order on territori es. However, for nearly all nests with extra-pair young, there were fe wer females synchronous with that nest on the cuckolder's territory th an on the territory of the cuckolded male. This ''advantage'' of a syn chrony difference was less pronounced for older males that cuckolded y ounger males, particularly when the two males were not neighbors. Coll ectively, these results suggest that breeding synchrony affects extra- pair mating by affecting mate guarding, but that breeding synchrony al one can not be used to predict which females are more likely to engage in extra-pair mating, nor with which extra-pair males they will mate. Understanding why extra-pair mating by older males is less affected b y breeding synchrony may explain much about both the proximate and ult imate causes of extrapair mating in red-winged blackbirds.