Uropygial gland

The uropygial gland, informally known as the preen gland, is a gland found in the large majority of birds that secretes an oil (preen oil) that birds use for preening. The chief compounds of preen oil are diester waxes called uropygiols.

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, kiwi, ostrich, and bustard. These typically find other means to stay clean and dry, such as taking a dust bath. See also powder down.

Waterproofing effect
The uropygial gland is strongly developed in many waterbirds, such as ducks (but not in cormorants which are also highly aquatic). It appears that the waterproofing effect is not primarily by the uropygiols – although they are hydrophobic – but by applying an electrostatic charge to the oiled feather through the mechanical action of preening.

Antiparasitic effect
An in vitro study suggests that the preen oils are effective against lice. Furthermore, the taxonomic richness of avian louse burdens covaries positively with preen gland size (relative to body size) across avian taxa suggesting coevolution between gland size and parasite biodiversity. Moreover, hoopoe preen gland harbours symbiotic bacteria whose excretions reduce the activity of feather-degrading bacteria and thus help to preserve the plumage.

Compare with

 * Sebaceous gland