What is your bird doing when he or she seems to be biting or chewing the base of his or her tail?
The uropygial gland can be found in many bird species, but today we'll be exploring its presence in parrots (psittacine birds).
Where is it and what does it look like? This skin gland is located at the dorsal base of the tail, above the pygostyle (Cooper and Harrison 1994). The gland is bilobed and opens from the skin onto a papilla surrounded by a small tuft of down feathers commonly referred to as the uropygial wick, or wick feathers. This tuft may help in collecting oil from the gland for distribution onto the feathers when preening (Dyce et al. 2002).
Who has one? Most parrots (order Psittaciformes of class Aves) have a uropygial gland. Amazon parrots (genus Amazona sp.) lack this gland, as well as the Palm Cockatoo (Probosciger aterrimus). A few macaw species such as the Hyacinth (Anodorhynchus hyacinthinus), Lear's (Anodorhynchus leari), and Spix's (Cyanopsitta spixii) also lack this gland (Doneley 2016).
What does it do?
Now, that's a loaded question! The uropygial gland produces preen oil, a fluid containing lipids, waxes, fatty acids, and some cells that serves many purposes. This is a holocrine gland (Dyce et al. 2002), meaning the cells lining the gland produce their lipid product within the cytoplasm, then upon cell death (rupture) they release their "product" into the gland lumen. The cells lining the gland are constantly being replaced so that a steady supply of oil is produced.
There are multiple functions of this gland:
Waterproofing: Spread by preening, the oil maintains feather condition and provides a hydrophobic effect (Jacob and Ziswisler 1982, Cooper and Harrison 1994). This is why water beads off of healthy, recently preened plumage (in addition to the feather barbules "zippering" together in an interlocking pattern).
Communication: Some species appear to communicate chemically via uropygial secretions. Dark-eyed Juncos (Junco hyemalis, a member of Passeriformes, the order often referred to as songbirds) have symbiotic bacteria that produce olfactory cues for reproduction in their preen gland (Whittaker et al. 2019). Olfactory cues have also been identified in the domestic chicken (Gallus gallus domesticus, members of Galliformes) (Hirao et al. 2008). Furthermore, preen oil may affect plumage colouration. Moreno-Rueda (2016) found that male House Sparrows (Passer domesticus, another passeriform) alter the colouration of their bib (a dark patch of feathers below the chin) and this may be related to advertising the individual's health status. Another study (Pérez-Rodriguez et al. 2011) looked at the rectrices (tail feathers) of the Bohemian Waxwing (Bombycilla garrulus) and applied oils from Black-billed Magpies (Pica pica) or Eagle Owls (Bubo bubo) and the result was a reduced brightness but increased ultraviolet hue (ultraviolet light is part of the visible spectrum in birds) and yellow chroma (the intensity of colour). While this is a sort of "Franken-study" situation, it suggests that preen oils may contribute to increased intensity of colouration in some birds. A similar study in House Finches (Carpodacus mexicanus) found comparable results (Lopez-Rull et al. 2010).
Antimicrobial effect: Some evidence suggests that preen oil may protect birds against bacteria that could degrade feather quality. Martin-Platero and associates (2006) tested an isolate from the preen oil of the Eurasian Hoopoe (Upupa epops, these birds belong to the order Bucerotiformes, shared with hornbills) and found in vitro that it seems to have broad-spectrum antibacterial activity. Similarly, Law-Brown (2001) found that isolates from preen oil of the Red-billed Wood Hoopoe (Phoeniculus purpureus) had antimicrobial activity in vitro and was also causing an aversive reaction upon ingestion in cats and monitor lizards, suggesting an antipredator mechanism. Similarly, a 2018 study (Braun et al.) examined preen gland secretion from the turkey (Meleagris gallopavo) and found some antimicrobial activity, also in vitro. Another study in House Sparrows (Passer domesticus) where the preen gland was experimentally removed demonstrated that these birds experienced an increase in the amount of bacteria on their feathers, however they were suspected to be nonpathogenic (not disease-causing) bacteria (Czirják et al. 2013). These few studies are just a handful of the work that's been done looking at the antimicrobial capacity of preen oil: There's a lot of conflicting evidence out there. Remember that birds are one of the most morphologically diverse groups of vertebrates with an estimated 9,000-10,000 species (Barrowclough et al. 2016), so what may be found in one species, genus, or family may not be demonstrated in another. In vitro studies (performed in a laboratory, sort of like saying "test tube" studies) are helpful and important, but not quite the same as in vivo studies in living organisms where there are so many more variables (making things much more complicated, as well as bringing ethical considerations to the table). Work on the antimicrobial efficacy of preen oil in parrots is basically non-existent.
Vitamin D: Vitamin D is effectively a hormone that regulates two important minerals, calcium and phosphorus (WebMD 2018). It has been speculated that preen oil contains provitamin D (7-dehydrocholesterol), that when spread onto the feathers would react with ultraviolet light (natural sunlight, or artificial broad spectrum lighting) and form vitamin D3 that would then be ingested during subsequent preening (Macwhirter 2006). Some older experiments exist that demonstrate that feathers can be a vehicle for vitamin D (Hou 1929, 1930a, 1930b, 1931; Rowan 1928), however newer primary literature is lacking on this subject. Parrots can absorb vitamin D3 from the diet, whether it comes from preen oil or a fortified formula (Klasing 1998). It has been demonstrated in a 2006 study using African Grey Parrots (Psittacus erithacus erithacus) that exposure to appropriate wavelengths of UV radiation increased blood concentrations of calcium as well as vitamin D (Stanford 2006). however it was not differentiated in this study where vitamin D3 synthesis was occurring (at preen oil on the feathers, or exposure to bare skin).
There is an excellent review of the studies performed on preen oil secretions by Gregorio Moreno-Rueda from 2017 that can be found here. Our understanding of the function of the gland is still in its infancy.
What can go wrong?
Now, that's a loaded question! The uropygial gland can be the subject of several types of disorders.
Vitamin A deficiency, unfortunately a common syndrome in companion parrots, can result in impaction of the uropygial gland. The lining or covering of any body surface is an epithelium, and depending on the body part, epithelial cells vary in form and function. The histologic structure of the uropygial gland is really well-described in Harem et al. 2010, the model species here was the Osprey (Pandion haliaetus, members of Accipitriformes, the order including all birds of prey except falcons), and we can probably assume that the structure is similar in most birds. When a bird's diet is vitamin A deficient, the normal epithelial lining of the uropygial gland develops squamous metaplasia: The lining is replaced by stratified squamous epithelial cells. This is a noncancerous process, but results in impaired gland function, and these new stratified squamous cells can "plug up" the gland (impaction). This also predisposes the gland to abscessation (Harrison et al. 2006). Please note that this is not the only sequela of a vitamin A deficient diet: The Merck Veterinary Manual lists the tissues affected by squamous metaplasia, including the oropharynx, choana, sinuses, digestive tract, urogenital tract, reproductive tract, in addition to the uropygial gland, as well as causing hyperkeratosis of the scaly skin of the legs and feet (Hoppes 2015). Hyperkeratosis differs from squamous metaplasia, as it involves a thickening of the outermost layer of skin (the stratum corneum, made up of layers of dead, keratinized cells), instead of replacing the normal epithelial cells by stratified squamous epithelial cells. Vitamin A deficiency is serious, and sadly, extremely common in captive psittacines. The biggest culprit is reliance on a poorly varied, seed-based diet (Harrison et al. 2006). Even naturally granivorous (seed-eating) birds will fail to thrive on commercially-produced seed-based diets, as products are often sold in an immature state of growth (being less nutritious compared to mature seeds) and do not contain the same nutrient profile as wild seeds (Roudybush and Grau 1985). Even mixing nutritionally-balanced fortified diets (such as pellets or extruded diets) with seeds will result in deficiencies, as it has been demonstrated that parrots do not select a balanced diet themselves, and instead eat what tastes good (Carciofi et al. 2003, Kollias 1995, Ullrey et al. 1991) and that preference can even vary between individuals of the same species (Carciofi et al. 2006).
Uropygial gland neoplasia is well-documented, particularly in common companion parrot species. For pet owners, what are we talking about when we're referring to neoplasia? Neoplasia is any uncontrolled, abnormal growth of any tissue in the body that is not under physiologic control by the body. When these cells are organized in a mass, we call this a tumour (Wikipedia 2021). It's a common misconception that "cancer" is one disease: It's actually hundreds of different types of abnormal cell growth that fall under the umbrella of neoplasia. Neoplasia can be benign, meaning it does not spread through the body. It can also be malignant, meaning the abnormal cells spread through the body (this process is called metastasis) and this "class" of neoplasia is what we classically call "cancer." Note that benign masses can still be problematic even if they do not spread: They may interfere with normal bodily functions depending on their location, and they may grow so large that they are a burden on the animal. Some benign masses can grow large enough that they "lose" their blood supply, which causes tissue death (a chunk of necrotic, or dead, tissue attached to the body is a very problematic situation and there are many possible sequelae of this). Biopsy, either incisional (where a sample of the mass is taken) or excisional (where the entire affected tissue is removed) is necessary to both determine the type of neoplasia and whether the surgical excision is complete. For pet owners, this is a discussion that should be had with your avian veterinarian regarding the best diagnostic option for your companion parrot. Adenomas, adenocarcinomas, and squamous cell carcinomas are probably the most common neoplasias of the uropygial gland, known to occur in many species of parrots (Beaufrere et al. 2008; Latimer 2006; Petrak and Gilmore 1982; Reece 1992). There are excellent review articles available, including Leach 1992, Reavill 2004, and Reece 1992. For pet owners, here is a brief description of these different types of neoplasias that affect the uropygial gland:
Adenomas are benign neoplasias of glandular epithelial cells. Recall that epithelial cells are the units that line body surfaces. While adenomas do not "spread" since they are benign, they can "transform" into adenocarcinoma.
Adenocarcinomas are malignant neoplasias of gladular epithelial cells.
Squamous cell carcinomas occur at squamous epithelial cells (flat cells that line body surfaces). This type of cancer is locally invasive, meaning that it "infiltrates" other local tissues, and can readily metastasize to other body organs.
In conclusion...
The uropygial gland is a fantastic piece of anatomy, serving many functions that we do not completely understand. Good nutrition helps to prevent husbandry-related disease of the gland. Unfortunately, this gland can develop some dangerous types of cancers. As birds are masters at hiding signs of illness, often until disease has progressed considerably, small changes in your bird's habits and appearance can be significant. Talk to your avian veterinary team if you have any concerns about clinical signs you may be seeing in your pet.
References:
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