Newly hatched and blind, bare and vulnerable, a baby songbird quickly transforms into a feathered, sharp-eyed creature capable of leaving the nest under its own power. Nestlings gain weight, grow feathers, and develop muscle control in mere days. Their begging behavior — vocalizing and showing a brightly colored, wide-open mouth — is finely tuned to attract parents who focus on stuffing food items into waiting mouths. Successfully securing food goes a long way to ensuring a nestling can grow, fledge and survive into adulthood.
The Common Cuckoo, a brood parasite which lays eggs in host species’ nests for them to raise, were studied by researchers investigating how host parents respond to chicks’ begging calls. Cuckoo nestlings typically outperform their nestmates with “supernormal stimulus” begging calls, attracting extra parental attention to themselves. Through playback experiments, the team found that Barn Swallow host parents could differentiate nestling begging calls and adjust feeding in response. What counts as an irresistible "feed me!" signal for one species may register as suspicious noise for another — a reminder that nestling begging is a coevolved language between parents and their young.