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Migrating blackcaps huddle at night at stopover sites

Small migratory birds employ a wide range of behavioral and physiological strategies to accelerate their fuel accumulation rate at migratory stopovers. Although they could potentially reduce the cost of night‐time rest by huddling with conspecifics, to date only anecdotal information exists about this behavior. We found that resting blackcaps (Sylvia atricapilla) regularly huddled in groups of two to six individuals in aviaries during migratory stopover. This occurred in spring migration in the Negev Desert and in autumn migration in central Poland. On some nights, huddling blackcaps used rest‐phase hypothermia, but on other nights, huddling birds maintained higher body temperatures at night than solitarily roosting individuals. Since migrating birds congregate at migratory stopovers, we anticipate huddling to be a common energy saving phenomenon among small migrating passerines.