Friends give benefits: autumn social familiarity preferences predict reproductive output

Fission-fusion dynamics create social instability, as individuals must adjust to changes in group size and composition. In many social species, group changes are associated with increases in aggression, stress responses and individual mortality. It has been hypothesized that fission-fusion processes select for strong bonds between familiar individuals that provide a predictable social environment across group changes. In the present study, I explored whether familiar social networks remain predictable across periods of social instability in brown-headed cowbird, Molothrus ater, flocks, and whether females who sustain stronger autumn familiarity preferences show higher reproductive output during the spring. During autumn, the organization of familiar social networks remained predictable across a series of introductions with novel flocks. Familiar individuals were able to maintain their relationships with each other despite large-scale group perturbations. During the spring, I found that autumn familiarity preference was the only predictor of reproductive output, with female cowbirds that sustained the strongest familiarity preferences laying more eggs than other females. These findings suggest that familiarity preferences have a cascading influence on later reproductive performance, and that the social dynamics of fission-fusion groups select for a familiarity-based social organization.