During their first 6 months, infants will often coo or babble while their caregivers are talking to them. By 7 to 8 months babies are more likely to show vocal turn taking, remaining quiet while someone speaks to them and making their own vocal responses when the person finishes. Babies may learn this type of turn taking from adults, who usually speak to a baby, wait for a coo or gurgle in response, and then speak again to the baby.
Rosenthal, M. K. (1982). Vocal dialogues in the neonatal period. Developmental Psychology, 18, 17
Snow, C. E., & Ferguson, C. A. (Eds.). (1977). Talking to children. Cambridge, UK: Cambridge University Press.