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 wick 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".
 Distribution amongst species
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. 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. See also powder down. Researchers have been unable to correlate the presence or absence of the uropygial gland with factors such as distribution, climate, ecology, or flightlessness.
 Gland secretion
The uropygial gland secretes an oil (preen oil) through the dorsal surface of the skin via a grease nipple-like nub or papilla. The oil contains a complex and variable mixture of substances formed greatly of aliphatic monoester waxes, formed of fatty acids and monohydroxy wax-alcohols. However, some types of diester waxes called uropygiols and containing hydroxyfatty acids and/or alkane-diols exist in the secretions of the uropygial gland of some groups of birds.
A bird will typically transfer preen oil to its body during preening by rubbing its beak and head against the gland opening and then rubbing the accumulated oil on the feathers of the body and wings, and on the skin of the feet and legs. Tailward areas are usually preened utilizing the beak, although some species, e.g. budgerigars use the feet to apply the oil to feathers around the vent.
Emperor Frederick II, in his thirteenth century treatise on falconry, was seemingly the first to discuss the function of the uropygial gland of birds. He believed that its product not only oiled the plumage but also provided a poison which was introduced by the claws of hawks and owls thus bringing quicker death to their prey. However, studies in 1678 on the question of the toxic nature of the uropygial gland secretion found no evidence to support Frederick’s contention.
Several researchers have reported differences in the relative gland weights attributing them to factors like seasonal changes, habitat, body weight, inter-individual variations, and gender. Significant differences are found in the relative gland size between males and females in most species, however, no coherent explanation has as yet been found for these results. Many ornithologists believe the function of the uropygial gland differs among various species of birds.
 Feather and body integrity
The preen oil is believed to help maintain the integrity of the feather structure. In waterbirds, the oil maintains the flexibility of feathers and keeps feather barbules from breaking. The interlocking barbules, when in good condition, form a barrier that helps repel water (see below). In some species, preen oil is also believed to maintain the integrity of the horny beak and the scaly skin of the legs and feet. It has also been speculated that in some species, the oil contains a precursor of vitamin D; this precursor is converted to vitamin D by the action of sunlight and then absorbed through the skin.
 Courtship and pheromone production
Some researchers have postulated that the change in preen oil viscosity may be related to the formation of the more brilliant plumage required for courtship, although later research did not find support for this idea. The results of other studies suggest that the gland in females may be involved in the production and secretion of lipids with female pheromone activity.
 Waterproofing effect
The uropygial gland is strongly developed in many waterbirds, such as ducks, petrels, pelicans and in the osprey and the oilbird (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. A study examining the gland's mass relative to body weight in 126 bird species showed the absence of a clear cut correlation between the gland's mass and the degree of birds' contact with water.
 Antiparasitic effect
The taxonomic richness of avian louse burdens covaries positively with uropygial gland size (relative to body size) across avian taxa suggesting coevolution between gland size and parasite biodiversity. The hoopoe (Upupa epops) uropygial gland harbours symbiotic bacteria whose excretions reduce the activity of feather-degrading bacteria and thus help to preserve the plumage.
In vitro studies suggest that the preen oils of Rock doves (Columba livia) are effective against lice, however in vivo studies which removed the preen gland from captive birds had no significant effect on louse loads over the course of four months (but did reduce plumage quality), suggesting that preen oil may not be an important defense against lice in this species. Similarly, there was no evidence to support a role of the gland secretion in attracting biting midges and black flies in Blue tits (Cyanistes caeruleus) and feral pigeons (Columba livia).
 See also
- Salibian, A. and Montalti, D. (2009). "Physiological and biochemical aspects of the avian uropygial gland". Brazilian Journal of Biology 69 (2): 437–46. doi:10.1590/S1519-69842009000200029. PMID 19675950.
- "BirdChannel.com". Retrieved August 10, 2012.
- Montalti, Diego and Salibian, Alfredo (2000). "Uropygial gland size and avian habitat". Ornitologia Neotropical 11 (4): 297–306.
- Moyer, BR; et al. (2003). "Experimental test of the importance of preen oil in Rock doves (Columba livia)". Auk 120 (2): 490–496. doi:10.1642/0004-8038(2003)120[0490:ETOTIO]2.0.CO;2.
- "The Grey Roost". Retrieved August 10, 2012.
- "Everything About Budgies". Retrieved August 10, 2012.
- Elder, William H. (1954). "The oil gland of birds". Wilson Bulletin 66 (1): 6–31.
- "Encyclopedia Britannica – Uropygial gland". Retrieved August 10, 2012.
- Møller, AP; et al. (2010). "Ectoparasites, uropygial glands and hatching success in birds". Oecologia 163 (2): 303–311. doi:10.1007/s00442-009-1548-x. PMID 20043177.
- Martin-Vivaldi, M; et al. (2009). "Antimicrobial chemicals in hoopoe preen secretions are produced by symbiotic bacteria". Proc. R. Soc. B 277 (1678): 123–30. doi:10.1098/rspb.2009.1377. PMC 2842625. PMID 19812087.
- Martínez-De La Puente, Josué; Rivero-De Aguilar, Juan; Cerro, Sara; Argüello, Anastasio; Merino, Santiago (2011). "Do secretions from the uropygial gland of birds attract biting midges and black flies?". Parasitology Research 109 (6): 1715–8. doi:10.1007/s00436-011-2436-y. PMID 21614548.