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uropygial gland[¦yu̇r·ə¦pij·ē·əl ‚gland]
(also preen gland, oil gland), a large, usually bilobed cutaneous gland in the majority of birds; its structure resembles that of the sebaceous glands. The uropygial gland is absent in a number of ratites, in bustards, and in some parrots and pigeons; it is highly developed in aquatic birds, especially the Procellariiformes and Pelecaniformes, and in the osprey. Located on the dorsal side at the base of the tail, it has from one to five (more often two) ducts, depending on the species. The gland’s secretion, which the bird applies to its plumage with its bill, contains lipoids mixed with proteins and inorganic salts; by maintaining the ordered structure of the plumage, the secretion prevents the feathers from getting wet. In the Procellariiformes, the musk duck, and the nestlings and incubating females of the hoopoe, the secretion of the gland has a sharp unpleasant odor, which apparently serves as protection against predators. In a number of birds, such as chickens and ducks, the uropygial gland secretion contains provitamin D—ergosterol— which the bird ingests as it preens its feathers.