Effects of experimental increase of corticosterone levels on begging behavior, immunity and parental provisioning rate in house sparrows

Begging is a complex display that is supposed to honestly indicate the need for food of nestlings, and, usually, parents use this information to adjust their investment in food provisioning. However, the mechanisms that ensure the honesty of begging as an indicator of need are still poorly known. It has been shown that levels of corticosterone (Cort), the hormone released during the stress response, raises during food shortage and is associated with increased begging rate. However, Cort also entails costs and these costs might prevent nestlings cheating. We tested this hypothesis in nestlings of the house sparrow. We experimentally increased levels of circulating Cort and investigated (1) the behavioral responses of nestlings, (2) the parental allocation of food and (3) the consequences on nestling growth and immune response. We found that Cort significantly increased begging rate but did not affect posture and position in the nest. Surprisingly, when begging effort was controlled statistically, control nestlings received more food from parents than Cort-treated nestlings. We also found that nestlings injected with Cort showed a weaker immune response and had lower body mass than controls. We suggest that Cort might have affected multiple aspects of nestling signaling, such as mouth color, and that parents use these multiple signals to adjust their feeding effort.