Nestlings beg to parents to communicate their need. Nevertheless, the specific signal driving parental care remains only partially understood. No study to date has been able to link a specific change in the physiological state of the young with, on the one hand, the modulation of a precise component of its begging behavior and, on the other hand, a direct modification of parental behavior reflecting an adjustment to an appropriate level of care. Here we orally administrated either exogenous corticosterone or a peanut oil control to free-living zebra finch nestlings and recorded begging behavior directly after treatment. Using a continuous automated monitoring system to record parental behavior in the wild, we simultaneously monitored the rate and duration of parental nest visits and foraging behavior at artificial feeders during 6 days posttreatment. We show that corticosterone modified the begging calls’ spectrum. Parents of corticosterone-treated broods spent more time in the nest and in feeders, and their older nestlings gained more body mass. Begging calls thus show a corticosterone-driven flexibility, which may inform parents of nestlings’ physiological state and allow them to provide an appropriate level of care.