The adaptive value of parental responsiveness to nestling begging

Despite extensive theoretical and empirical research into offspring food solicitation behaviour as a model for parent-offspring conflict and communication, the adaptive value of parental responsiveness to begging has never been tested experimentally. Game theory models, as well as empirical studies, suggest that begging conveys information on offspring state, which implies that parental investment can be better translated to fitness by responding to begging when allocating resources rather than by ignoring it. However, this assumption and its underlying mechanisms have received little or no attention. Here we show by experiments with hand-raised house sparrow (Passer domesticus) nestlings that a 'responsive parent' will do better than a hypothetical 'non-responsive' mutant (that provides similar food amounts, but irrespective of begging). This is neither because food-deprived nestlings convert food to mass more efficiently, however, nor because responsiveness reduces costly begging. Rather, responsiveness to begging is adaptive because it reduces two opposing risks: one is wasting time when returning too soon to feed already satiated nestlings and the other is repeatedly overlooking some nestlings as a result of the stochastic nature of a random, non-responsive strategy. This study provides the first experimental evidence for the adaptive value of parental responsiveness to offspring begging.