Dynamic membership drives long-term persistence of sparrow winter social communities

Social groups range from highly stable to highly dynamic over extended periods. Demographic factors like individual lifespans and rates of births, deaths, immigration, and emigration in a population can strongly impact group stability and persistence over time. Highly stable long-term groups are often driven by long-lived core individuals that maintain relationships, kin-structure, or high philopatry. In opposition, fluid groups of individuals that associate repeatedly, called social communities, can persist in populations that have high turnover, lack kin structure, and exhibit high fission-fusion dynamics. However, we lack an understanding of how animal social communities persist amidst highly dynamic membership. We investigated the drivers of persistence of social communities in a winter population of migratory golden-crowned sparrows over 12 years. Sparrows frequently flock with the same individuals over the entire winter, forming social communities that are identified from discrete clusters within a social network. Long-term community membership was highly dynamic due to high annual turnover and the relatively short lifespans of individuals. However, the social fidelity of returning members and recruitment of new members between consecutive years resulted in communities that persisted for multiple years, even when there was a complete turnover of members over time. Our results indicate that groups can persist with consistent short-term membership, overlapping generations, and turnover with balanced rates of recruitment and loss. This dynamic stability of individual membership may be a universal feature of large, long-term groups found in many social systems, from animal societies with fission-fusion dynamics to networks of human collaboration.