The uropygial gland, informally known as the preen gland or the oil gland, is a bilobed

sebaceous gland A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in ...

possessed by the majority of birds used to distribute the gland's oil through the plumage by means of preening. It is located dorsally at the base of the tail (between the fourth caudal vertebra and the

pygostyle Pygostyle is a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main component o ...

) 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 Wick most often refers to: * Capillary action ("wicking") ** Candle wick, the cord used in a candle or oil lamp ** Solder wick, a copper-braided wire used to desolder electronic contacts Wick or WICK may also refer to: Places and placenames ...

for the preen oil (see below). It is a

holocrine Holocrine (from Ancient Greek ὅλος; ''hólos'', “whole, entire” + κρῑ́νω; ''krī́nō'', “to separate”) is a term used to classify the mode of secretion in exocrine glands in the study of histology. Holocrine secretions are p ...

gland enclosed in a connective tissue capsule made up of glandular

acini An acinus (; : acini; adjective, acinar or acinous) refers to any cluster of cells that resembles a many-lobed "berry", such as a raspberry (''acinus'' is Latin for "berry"). The berry-shaped termination of an exocrine gland, where the secret ...

that deposit their oil secretion into a common collector tube ending in a variable number of pores (openings), most typically 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).

Etymology

From :

Medieval Latin Medieval Latin was the form of Literary Latin used in Roman Catholic Church, Roman Catholic Western Europe during the Middle Ages. It was also the administrative language in the former Western Roman Empire, Roman Provinces of Mauretania, Numidi ...

, from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

(''ouropugion''), from (''oura'') 'tail' and (''puge'') 'rump'.

Distribution amongst species

The gland is invariably present during

embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...

, whereas it can be

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

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 The down of birds is a layer of fine feathers found under the tougher exterior feathers. Very young birds are clad only in down. Powder down is a specialized type of down found only in a few groups of birds. Down is a fine thermal insulator and p ...

for feather maintenance. These include kiwis (''Apterygidae''),

emu The emu (; ''Dromaius novaehollandiae'') is a species of flightless bird endemism, endemic to Australia, where it is the Tallest extant birds, tallest native bird. It is the only extant taxon, extant member of the genus ''Dromaius'' and the ...

(''Dromaiidae''),

ostriches Ostriches are large flightless birds. Two living species are recognised, the common ostrich, native to large parts of sub-Saharan Africa, and the Somali ostrich, native to the Horn of Africa. They are the heaviest and largest living birds, w ...

(''Struthionidae''), rheas (''Rheidae''),

cassowaries Cassowaries (; Biak language, Biak: ''man suar'' ; ; Papuan_languages, Papuan: ''kasu weri'' ) are flightless birds of the genus ''Casuarius'', in the order Casuariiformes. They are classified as ratites, flightless birds without a keel (bird a ...

(''Casuariidae''), mesites (''Mesitornithidae''), bustards (''Otididae''), pigeons and doves (''Columbidae''),

amazon parrot Amazon parrots are parrots in the genus ''Amazona''. They are medium-sized, short-tailed parrots native to the Americas, with their range extending from South America to Mexico and the Caribbean. ''Amazona'' is one of the 92 genera of parrots t ...

s (''Psittacidae''),

frogmouths The frogmouths (Podargidae) are a group of nocturnal birds related to owlet-nightjars, swift (bird), swifts, and hummingbirds. Species in the group are distributed in the Indomalayan realm, Indomalayan and Australasian realms. Biology They are n ...

(''Podargidae''), and

woodpeckers Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar and the extreme polar regions. ...

(''Picidae''). These birds typically find other means to stay clean and dry, such as taking a

dust bath Dust bathing (also called sand bathing) is an animal behavior characterized by rolling or moving around in dust, dry earth or sand, with the likely purpose of removing parasites from fur, feathers or skin. Dust bathing is a Comfort behaviour in ...

. 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 Grease fitting on a bearing A grease fitting, grease nipple, Zerk fitting, grease zerk, Alemite fitting, or divit is a metal fitting used in mechanical systems to feed lubricants, usually lubricating grease, into a bearing under moderate t ...

-like nub or papilla. The oil contains a complex and variable mixture of substances formed greatly of

aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated (in which all ...

monoester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distinct ...

waxes, formed of

fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

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. Preen gland secretion of some, but not all, birds have shown to be

antimicrobial An antimicrobial is an agent that kills microorganisms (microbicide) or stops their growth (bacteriostatic agent). Antimicrobial medicines can be grouped according to the microorganisms they are used to treat. For example, antibiotics are used aga ...

. Some birds harbor bacteria in their preen gland, which to date, have (exclusively) been isolated from preen glands (e.g. ''

Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are Gram-positive cocci that often occur in pairs ( diplococci) or short chains, and are difficult to distinguish from streptococci on physical ch ...

phoeniculicola'' and '' Corynebacterium uropygiale''). Some of those bacteria add to the antimicrobial properties of preen wax. A bird will typically transfer preen oil to its body during preening by rubbing its

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and ...

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, such as budgerigars, use the feet to apply the oil to feathers around the vent.

History

Emperor Frederick II Frederick II (, , , ; 26 December 1194 – 13 December 1250) was King of Sicily from 1198, King of Germany from 1212, King of Italy and Holy Roman Emperor from 1220 and King of Jerusalem from 1225. He was the son of Emperor Henry VI of the Ho ...

, 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 Hawks are bird of prey, birds of prey of the family Accipitridae. They are very widely distributed and are found on all continents, except Antarctica. The subfamily Accipitrinae includes goshawks, sparrowhawks, sharp-shinned hawks, and othe ...

and

owls Owls are birds from the Order (biology), order Strigiformes (), which includes over 200 species of mostly Solitary animal, solitary and Nocturnal animal, nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vis ...

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.

Function

Several researchers have reported differences in the relative gland weights attributing them to factors like seasonal changes, habitat, body weight, inter-individual variations, and sex. 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

Preen oil is believed to help maintain the integrity of

feathers Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on both Bird, avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in ...

. In waterbirds, preen 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, preen oil contains a precursor of

vitamin D Vitamin D is a group of structurally related, fat-soluble compounds responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions. In humans, the most important compo ...

; 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 Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

may be related to the formation of the more brilliant plumage required for

courtship Courtship is the period wherein some couples get to know each other prior to a possible marriage or committed romantic, ''de facto'' relationship. Courtship traditionally may begin after a betrothal and may conclude with the celebration of marri ...

, 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 Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...

with female

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

activity.

Waterproofing effect

The uropygial gland is strongly developed in many waterbirds, such as

ducks Duck is the common name for numerous species of waterfowl in the family (biology), family Anatidae. Ducks are generally smaller and shorter-necked than swans and goose, geese, which are members of the same family. Divided among several subfam ...

petrel Petrels are tube-nosed seabirds in the phylogenetic order Procellariiformes. Description Petrels are a monophyletic group of marine seabirds, sharing a characteristic of a nostril arrangement that results in the name "tubenoses". Petrels enco ...

pelican Pelicans (genus ''Pelecanus'') are a genus of large water birds that make up the family Pelecanidae. They are characterized by a long beak and a large throat pouch used for catching prey and draining water from the scooped-up contents before ...

s and in the

osprey The osprey (; ''Pandion haliaetus''), historically known as sea hawk, river hawk, and fish hawk, is a diurnal, fish-eating bird of prey with a cosmopolitan range. It is a large raptor, reaching more than in length and a wingspan of . It ...

and the

oilbird The oilbird (''Steatornis caripensis''), locally known as the , is a bird species found in the northern areas of South America including the Caribbean island of Trinidad. It is the only living species in the genus ''Steatornis'', the family Stea ...

. 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. Anecdotal reports indicating that the waterproofing effect of the

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

uropygiols might be increased by

electrostatic Electrostatics is a branch of physics that studies slow-moving or stationary electric charges. Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word (), mean ...

charge to the oiled feather through the mechanical action of preening are not supported by scientific studies.

Antiparasitic effect

The taxonomic richness of avian louse burdens covaries positively with uropygial gland size (relative to body size) across avian taxa, suggesting

coevolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well a ...

between gland size and parasite

biodiversity Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...

. The

hoopoe Hoopoes () are colourful birds found across Africa, Asia, and Europe, notable for their distinctive "Crest (feathers), crown" of feathers which can be raised or lowered at will. Two living and one extinct species are recognized, though for many y ...

(''Upupa epops'') uropygial gland harbours

symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

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 dove The rock dove (''Columba livia''), also sometimes known as "rock pigeon" or "common pigeon", is a member of the bird family Columbidae (doves and pigeons). In common usage, it is often simply referred to as the "pigeon", although the rock dov ...

s (''Columba livia'') are effective against

lice Louse (: lice) is the common name for any member of the infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an order, until a 2021 genetic study determined th ...

, 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 tit The Eurasian blue tit (''Cyanistes caeruleus'') is a small passerine bird in the tit (bird), tit family, Paridae. It is easily recognizable by its blue and yellow plumage and small size. Eurasian blue tits, usually resident bird, resident a ...

s (''Cyanistes caeruleus'') and

feral pigeon Feral pigeons are birds derived from domesticated populations of the rock dove ''Columba livia'', descendants that have escaped and are living independently from (and often unwanted by) humans, having gone "feral". They are sometimes given the ...

s (''Columba livia'').

Cosmetic effect

Secretions of the uropygial gland of

greater flamingo The greater flamingo (''Phoenicopterus roseus'') is the most widespread and largest species of the flamingo family. Common in the Old World, they are found in Northern (coastal) and Sub-Saharan Africa, the Indian Subcontinent (south of the Him ...

s (''Phoenicopterus roseus'') contain

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

s, organic pigments which give flamingos their pink colour. During the breeding season, greater flamingos increase the frequency of their spreading uropygial secretions over their feathers and thereby enhance their colour. This cosmetic use of uropygial secretions has been described as applying "make-up".

Chemical composition

The biochemical composition of preen oil has been reported to significantly vary among birds in several ways: for example, the qualitative and quantitative content of the volatile compounds, chain length of diols, lipid concentration, ester saturation, type of fatty acids and their percentage composition. This variation is due to a range of factors, including species, age, sex and season. In chickens, a limited number of studies have investigated the specific effect of diet on the fatty acid composition of preen oil in meat chickens. The preen oil of meat chickens is dominated by saturated fatty acids, which make up to 97% of the total fatty acids, with 13 different medium to long chain saturated fatty acids (C8:0 to C22:0) detected. The preen oil contained several odd-chain fatty acids, which suggests they may be derived from lipolysis by the uropygial gland and/or its microbiome. Diet and gender had small but significant effects on levels of specific saturated fatty acid in the preen oil. The fatty acid composition of the preen oil did not reflect the more diverse fatty acid compositions of the diet or whole blood. Therefore, this clearly indicate that measuring the fatty acid profile of preen oil is not a suitable alternative approach for predicting the fatty acid composition of the blood of meat chickens.

References

External links

Diagram of uropygial gland structure
{{DEFAULTSORT:Uropygial Gland Bird anatomy