A large gland at the base of a bird's tail dorsal to the cloaca at the end of the pygostyle that secretes an oil used in preening. It opens through a caudally directed nipple.
Comment:
The gland is found near the base of the tail and is shaped into two symmetric parts. The oil of each part of the gland is secreted through the surface of the skin through a grease nipple-like nub. A bird will typically transfer this oil to its feathers by rubbing its head against the oil and then around the rest of the body. Tailward areas are usually preened utilizing the beak. Not all birds have a uropygial gland. Exceptions include the emu, ostrich, and bustards. These typically find other means to stay clean and dry, such as taking a dust bath