Genetic monogamy in blue-headed vireos and a comparison with a sympatric vireo with extrapair paternity

Based on the breeding synchrony hypothesis, we predicted, in two congeners that nest in similar habitat but differ in nesting synchrony, that blue-hea ded vireos (Vireo solitarius) would have fewer extrapair fertilizations (EP Fs) than red-eyed vireos (V. olivaceus). EPFs were rare in blue-headed vire os (1/37 nestlings), but common in red-eyed vireos (11/19 nestlings). We st udied the behavior of blue-headed vireos to determine what factors could pr omote genetic monogamy. We found no evidence that males mace guarded to pre vent extrapair copulations from occurring. Males did not follow fertile mat es closely when mates left the nest (14-25% of female departures) and, duri ng the egg-laying period, males were often alone on the nest (22.3 min/h). Female blue-headed vireos, but not red-eyed vireos, obtain direct benefits from social mates such as nest building and incubation (49.1% of the total) , and they assess male quality long before becoming fertile. Female blue-he aded vireos spent more time incubating when their mates had low incubation effort. Furthermore, male incubation effort was positively correlated with nest survival during incubation. We discuss the evolution of genetic monoga my and sex role convergence in blue-headed vireos in relation to asynchrono us breeding.