Parent-absent begging: evidence for sibling honesty and cooperation in the spotless starling (Sturnus unicolor)

Begging in avian nestlings is a highly conspicuous behavior with important implications for the study of parent-offspring conflict. In some species, nestlings also call for long bouts in the absence of parents, and it has been proposed that this behavior is used by nestlings as a means of negotiating access to food. We studied this phenomenon in the spotless starling (Sturnus unicolor). We found that parent-absent calls were acoustically distinct from parent-present calls. Observations showed that the probability of parent-absent begging increased with nestling age and brood size, whereas it decreased with increasing body condition. This result was confirmed by an experiment that showed that nestlings produced higher parent-absent begging rates when food deprived than when satiated. Finally, we carried out a playback experiment to test the reaction of nestlings to parent-absent begging by fellow nestlings. Principle components analyses yielded 2 independent components of begging: 1) a general begging component and 2) a second factor that measures the relative contribution of communicative begging over competitive begging. Nestlings exposed to playback decreased their general begging levels and simultaneously increased the relative contribution of communicative over competitive begging. This behavior may favor needy nestlings to obtain impending feedings while keeping high levels of food solicitation from parents and is consistent with a cooperative strategy among nestlings. Future research should consider the actual response of parents to these signals.