Incubation delays territory defence by male blue-headed vireos, Vireo solitarius

Abstract

Animals often face trade-offs during reproduction between activities such as parental care and territory defence. In species that are socially monogamous, males and females are faced with the additional problem of coordinating their respective contributions to such activities. Here, we examined male incubation behaviour in a passerine that shows genetic monogamy and in which males incubate extensively. Male incubation is rare in North American passerines and little is known about how males defend their territories during the incubation stage and whether females compensate for lower male incubation during territorial intrusions. Blue-headed vireo males contribute 50% of incubation time during the day. We performed playbacks to simulate intrusions to males while they were on the nest and off the nest during the incubation period. On average, males took 18 min to arrive at the playback location while on the nest incubating, but took less than 2 min when not incubating. In addition, only 44% of the males sang while on the nest, whereas all males sang in response to the playback when off the nest. This result suggests that males probably delay territory defence until females return to the nest to avoid exposing the nest to predators. Females returned to the nest to relieve their mates from incubation duties sooner during experimental intrusions than during control periods, which also allowed males to pursue intruders sooner. Genetic monogamy may underlie this apparent cooperation and the sex-role convergence exemplified in this species.