Abstract
Nest attendance during incubation is characterized by an inequitable division of labor in house sparrows, Passer domesticus, with females spending more time at the nest than males. Previous research has shown that if male contributions are reduced experimentally via testosterone (T) implants, females compensate partially for those reductions, consistent with predictions from most models of negotiated biparental care. In this study, we attempted to identify the cues and contexts generating partial compensation, using data from both unmanipulated parents and pairs with T-males. Both males and females of this species sometimes leave the nest before their mate returns to relieve them, and we found that these unrelieved departures by unmanipulated individuals occur when partners are on lengthy recesses. Females compensated partially for long male recesses by marginally extending their bouts; most females also slightly reduced their next recess. By contrast, when males left before their mate returned, they left earlier than when they waited for the female. Neither males nor females adjusted their recess lengths after returning to the nest and discovering that their partner was absent. More pronounced changes in nest attendance of unmanipulated parents occurred in the context of 'visits', when individuals returned to the nest but then left without relieving their mate. Such visits effectively prolonged the bout of the on-duty partner and extended the visitor's recess. Analyses of behavior of T-males and their mates revealed that T-males had significantly longer recesses than control males, and that their mates, in turn, had elevated rates of unrelieved departures. T-males also visited their on-duty mates more often than control males, whereas female visits to T-males were rare. Collectively, the predicted changes in female nest attendance associated with lengthy male recesses and male and female visits account reasonably well for the compensatory response of females paired to T-males. The majority of female compensation was attributable to changes in visit behavior, however, suggesting that much of the negotiation over nest attendance in this species occurs during direct interactions between mates.
