A baby songbird doesn't emerge from the egg singing perfectly. It starts out babbling and gradually refines its tune over time. Human infants follow a similar developmental path when learning to talk. Scientists have thus often compared the acquisition of human speech to that of birdsong. But whether the mechanisms of vocal development are the same in humans and birds is a question few studies have tackled. To that end, new research should prove insightful. According to a report published online this week by the Proceedings of the National Academy of Sciences, babbling human babies respond to social cues from their mothers in much the way that avian babies do.

Previous efforts to understand how babies learn to talk had focused on the role of imitation in speech acquisition. In the new work, psychologist Michael H. Goldstein of Franklin and Marshall College and his colleagues turned their attention to social interactions between eight-month-old infants and their mothers. The researchers directed the mothers to act in certain ways while responding to their baby's utterances during 30-minute play sessions. "The mothers did not change how they talked but whether they touched or smiled at the baby," team member Meredith West of Indiana University explains. Like songbird chicks, the investigators found, the babies registered the social consequences of sound-making and adjusted their babbling accordingly.

West describes the findings as the first to show "that babies change how they vocalize in response to social responses--not sounds, but sights--by using more mature sounds." Remarks Goldstein: "This project shows that maternal behavior and infant sensory capacities interact to generate the development of more advanced infant behavior. It shows that social learning is a crucial part of vocal development."