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Bird vocalisation
Flight calls are vocalisations made by birds in flight, often serving to keep flocks together.
References
Cech, Rick; Dunning, John B. Jr.; Elphick, Chris (2001). "Behavior". In Elphick, Chris; Dunning, John B. Jr.; Sibley, David Allen (eds.). The Sibley Guide to Bird Life and Behavior. New York: Alfred A. Knopf. pp. 68–69. ISBN978-1-4000-4386-6.