The formation of social relationships in complex groups is critical in shaping patterns of social organization and behavioral development. In many birds, young individuals remain dependent on their parents for extended periods but must abruptly transition to navigating interactions in the wider group after independence. While lack of social relationships during this period is detrimental in the development of later social skills, little is known about the social relationships juveniles form after independence in many bird species. In this study, we describe patterns of social interactions in juvenile Gouldian finches (Erythrura gouldiae) during transitions from family groups into flocks of unfamiliar individuals. Twenty juveniles from four families were introduced into two flocks. After introductions, juveniles showed a gradient of approach rates with most approaches directed toward siblings, followed by juvenile peers, adult females, and lastly adult males. Significant preferences for siblings resulted in the emergence of sibling subgroups within the larger social network. This intentional self-assortment of siblings suggests that sibling subgroups are an important bridge linking social connections within the family and the wider group. Such findings suggests that sibling relationships have a critical role in the socialization after independence, as well as structuring the social organization of Gouldian finch flocks.

The organization of animal groups is both a cause of, and consequence of, patterns of interactions among individuals. The stability of animal social organization reflects how individuals construct and maintain resilient patterns of interactions across changes in group size and membership. In this study we describe patterns of social interactions in captive flocks of Gouldian Finches (Erythrura gouldiae) that were exposed to changing social conditions. A flock of adult Gouldian Finches was exposed to two changing conditions, an introduction condition where juveniles were introduced to the flock, and a fission-fusion condition where the flock was split into two smaller flocks and a new group of juveniles introduced to each. We show that the social organization of captive Gouldian finch flocks is characterized by stable homophilic communities of adult females and juveniles. Females showed higher rates of approaches to other adult females, while juveniles maintined higher rates of approaches towards other juveniles. These findings highlight how the stable interactive decisions made by individuals contribute to higher-order patterns of organization in bird flocks. Furthermore, this study is one of the few to demonstrate self-assortment of juveniles in passerine flocks and suggests that peer and sibling interactions are an unexplored area in avian social development.