Juvenile Gouldian finches (Erythrura gouldiae) form sibling subgroups during social integration.

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.