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Uropygial gland

Uropygial gland

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The uropygial gland, informally known as the preen gland, is a gland
A gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...

 found in the large majority of birds that secretes an oil (preen oil) that birds use for preen
- Birds :*Preen, personal grooming of a bird's feathers especially by using its beak*Preen gland, also called the uropygial gland, an oil gland found in many bird species*Preen oil, an oil made by the uropygial gland found in many bird species...

ing. The chief compounds of preen oil are diester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

es 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
The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...

. Not all birds have a uropygial gland. Exceptions include the emu
The Emu Dromaius novaehollandiae) is the largest bird native to Australia and the only extant member of the genus Dromaius. It is the second-largest extant bird in the world by height, after its ratite relative, the ostrich. There are three subspecies of Emus in Australia...

, kiwi
Kiwi are flightless birds endemic to New Zealand, in the genus Apteryx and family Apterygidae.At around the size of a domestic chicken, kiwi are by far the smallest living ratites and lay the largest egg in relation to their body size of any species of bird in the world...

, ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...

, and bustard
Bustards, including floricans and korhaans, are large terrestrial birds mainly associated with dry open country and steppes in the Old World...

. These typically find other means to stay clean and dry, such as taking a dust bath
Dust bath
Dust bath is the act of an animal grooming and cleansing its skin or fur from parasites by rolling or moving around within dust or sand.-Overview:...

. See also powder down.

Waterproofing effect

The uropygial gland is strongly developed in many waterbirds, such as ducks (but not in cormorant
The bird family Phalacrocoracidae is represented by some 40 species of cormorants and shags. Several different classifications of the family have been proposed recently, and the number of genera is disputed.- Names :...

s 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
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

. Moreover, hoopoe
The Hoopoe is a colourful bird that is found across Afro-Eurasia, notable for its distinctive 'crown' of feathers. It is the only extant species in the family Upupidae. One insular species, the Giant Hoopoe of Saint Helena, is extinct, and the Madagascar subspecies of the Hoopoe is sometimes...

 preen gland harbours symbiotic bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

whose excretions reduce the activity of feather-degrading bacteria and thus help to preserve the plumage.