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Among shorebirds, many individuals make migratory and short‐distance movements in large flocks, suggesting that stable social groups may persist within populations for days to months. We examined high‐tide associations among individual radio‐marked Dunlin wintering at Humboldt Bay, California, to determine if flocks represented stable social groups. The rate of co‐occurrence of two individuals in the same flock was generally low (x̄= 15% of surveys), and 86% of pairs co‐occurred no more than expected by chance. Associations were ephemeral, lasting 1.1 consecutive high tides on average. Variation in co‐occurrence reflected individual fidelity to roosts, as well as population‐wide differences in space use with precipitation and time of day. Dunlin flock composition was fluid, and individuals appeared to associate by chance according to shared attraction to common roosts. Our findings are consistent with …