Zebra finches demonstrate selective affiliation between juvenile offspring and parents which, like affiliation between pair partners, is characterized by proximity, vocal communication and contact behaviors. In addition, they exhibit vocal learning, in which juvenile males learn courtship song through socially-guided feedback from adult tutors. This experiment tested the hypothesis that the nonapeptide arginine vasotocin (AVT, avian homologue of vasopressin) and the V1a receptor subtype (V1aR) play an organizational role prior to fledging in affiliative behavior and species-typical vocal learning. Zebra finch hatchlings of both sexes received daily intracranial injections (posthatch days 2-8) of either AVT, Manning Compound (MC, a V1aR antagonist) or a saline control. The social development of both sexes was assessed by measuring responsiveness to isolation from the family and attachment to the male parent. When the male subjects reached adulthood, crystalized song was recorded and compared to the father’s song. Profound differences were observed between the treatment groups on the first day following fledging (posthatch days 16-24). When reunited with their male parent following isolation, AVT subjects increased their activity level (perch hops and head saccades), whereas MC subjects decreased their activity level. Controls did not vary their activity level in response to the presence of the male parent. In addition, subjects in the different treatments also differed in the rate of long tonal calls. Subjects injected with MC vocalized less than both AVT and control birds. As adults, males injected with AVT produced a song that was a higher fidelity match to their father’s song than controls, whereas MC males produced a poorer match to father’s song. This suggests AVT and the V1aR play organizational roles in social development, perhaps modifying early attentiveness to social stimuli and attachment leading to downstream differences in learned socially-relevant behaviors.