Abstract
Migratory birds often form flocks on their wintering grounds, but
important details of social structure such as the patterns of association
between individuals are virtually unknown. We analysed networks of
co-membership in short-term flocks for wintering golden-crowned sparrows
(Zonotrichia atricapilla) across three years and discovered social
complexity unsuspected for migratory songbirds. The population was
consistently clustered into distinct social communities within a
relatively small area (~ 7 ha). Birds returned to the same community
across years, with mortality and recruitment leading to some degree of
turnover in membership. These spatiotemporal patterns were explained by
the combination of space use and social preference – birds that flocked
together in one year flocked together again in the subsequent year more
often than were expected based on degrees of home range overlap. Our
results suggest that a surprising level of social fidelity across years
leads to repeatable patterns of social network structure in migratory
populations.