An integrative understanding of the evolution of complex social behavior requires a framework that links insights about the ecological and phylogenetic context of behavior, with the molecular, neural, and developmental mechanisms that produce it. In order to provide insight into the mechanisms underlying complex adaptive social behaviors, I examined the proximate and developmental factors that contribute to species-typical social behaviors in a well-studied song bird, the zebra finch (Taeniopygia guttata). 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. I demonstrate that proximate factors-including age, breeding experience, and the social group-influence pairing, reproductive success, and the flexible use of alternative reproductive strategies in the zebra finch. Additionally, I present the results of an experiment testing the hypothesis that the nonapeptide arginine vasotocin (AVT, avian homologue of vasopressin) and nonapeptide receptors play organizational roles in the development of speciestypical affiliative behavior, courtship, and vocal learning. Zebra finch hatchlings of both sexes received intracranial injections (posthatch days 2-8) of AVT, Manning Compound (MC, a V1a receptor antagonist) or a saline control. I assessed affiliative behaviors using a series of behavioral assays throughout development. I demonstrate that manipulations of the AVT system early in life alter affiliative interest in parents and opposite sex conspecifics during juvenile development as well as vocal learning in males. I also provide the first evidence that AVT and nonapeptide receptors play organizational roles in both the development of pair bonding in adulthood and the neural substrate underlying these behaviors in a bird. Thus, my research provides support for the idea that the nonapeptides, which modulate the activity of neural circuits across different social contexts, may provide an important mechanism underlying both the evolution and the development of diverse social phenotypes across vertebrate taxa.