Preen gland

The uropygial gland, informally known as the preen gland or the oil gland, is a bilobate sebaceous gland possessed by the majority of birds. It is located dorsally at the base of the tail (between the fourth caudal vertebrae and the pygostyle) and is greatly variable in both shape and size. In some species, the opening of the gland has a small tuft of feathers to provide a for the preen oil (see below). It is a holocrine gland enclosed in a connective tissue capsule made up of glandular acini that deposit their oil secretion into a common collector tube ending in a variable number of pores (openings), most usually two. Each lobe has a central cavity that collects the secretion from tubules arranged radially around the cavity. The gland secretion is conveyed to the surface via ducts that, in most species, open at the top of a papilla (nipple-like structure).

From 'uropygium': Mediaeval Latin, from Ancient Greek οὐροπύγιον (ouropugion), from οὐρά (oura) "tail" and πυγή (puge) "rump".

The gland is invariably present at embryonic stages, whereas it can be vestigial in adults of certain orders, families, genera and species. Some or all species in at least nine families of birds lack a uropygial gland, mostly the ones unable to fly or the ones that produce powder down for feather maintenance. These include kiwis (Apterygidae), emu (Dromaiidae), ostriches (Struthionidae), rheas (Rheidae), cassowaries (Casuariidae), mesites (Mesitornithidae), bustards (Otididae), pigeons and doves (Columbidae), amazon parrots (Psittacidae), frogmouths (Podargidae), and woodpeckers (Picidae). These birds typically find other means to stay clean and dry, such as taking a dust bath. Researchers have been unable to correlate the presence or absence of the uropygial gland with factors such as distribution, climate, ecology, or flightlessness.

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