Avian Uropygial Gland: A Comprehensive Guide with Interactive Labelling Practice and Quiz
Uropygial Glands in Birds – Anatomy, Physiology, Diagrams, Practice & Quiz
Glossary
- Holocrine Secretion: A process of glandular secretion where the entire secretory cell disintegrates to release its accumulated products (lipids).
- Trabeculae: Connective tissue septa or "walls" that originate from the capsule and extend into the gland to provide structural support and divide it into lobes.
- Papilla: The small, nipple-like projection at the rear of the gland through which the oil is discharged.
- Preen Wick (Circlet): A specialized arrangement of small feathers (down) at the tip of the papilla that acts as a reservoir to hold oil for the bird's beak.
- Invagination: The folding in of a membrane or layer of tissue so as to form a pocket—essential in the development of the glandular ducts.
- 7-Dehydrocholesterol: A precursor sterol found in preen oil that converts to Vitamin D3 when exposed to ultraviolet (UV) sunlight.
- Lipid Monoesters: The primary chemical components of uropygial wax that provide the hydrophobic (water-repelling) properties to the plumage.
- Rectrices: The large, stiff feathers of the bird’s tail used for braking and steering, located just beneath the uropygial gland.
- Pygostyle: The final bone of the tail (fused vertebrae) which provides the anatomical anchor point for the gland and tail feathers.
- Bacteriostatic: The property of the gland's secretion that inhibits the growth of bacteria without necessarily killing them, maintaining feather integrity.
Uropygial gland or preen gland is a bilobed holocrine gland located on the rump immediately in front of the rectrices, at the base of the tail (uropygium) of birds.
- It is encapsulated by connective tissue.
- It functions as an accessory to feathers and is found in most birds.
- Highly developed in waterfowl.
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| Uropygial Gland and Surroundings Representation focusing on the boundary between the glandular tissue and the surrounding dermis, clearly demarcating the glandular lobes, the papilla, and the feather tuft. |
They are absent in ostriches, emus, cassowaries and a few flying birds such as bustards, some parrots, and some pigeons.
The largest glands are found in birds that swim, dive or rest in water, such as petrels, pelicans, ducks, and grebes.
This gland is prominent in African Greys and budgies but absent in many parrots (e.g. Amazon parrots), ostriches and many pigeons.
The lipids secreted by the gland and the epidermal cells are spread over the feathers by the bill during preening, and are supposed to help in
- keeping the feathers dry and pliable and
- also maintain buoyancy.
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| Macro-anatomical orientation of the uropygial gland in Columba Livia. The heart-shaped bilobed structure is situated dorsally at the base below the rectrices (tail feathers). Key features include a papilla which acts as an exit point for lipid secretions and a feather circlet, a ring of small feathers helping channelise preen oil to the bird's beak for grooming. |
Anatomy
The bilobed structure develops from a pair of invagination of the dorsal ectoderm of each side of the free coccygeal vertebrae, giving rise to numerous secretory tubules surrounded by a capsule of connective tissue without smooth muscle fibres of mesodermal origin.
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| Morphological relationship between the external position and internal architecture of the Uropygial gland The figure illustrates the external dorsal view (left) to the internal transverse section (right) of the Uropygial gland. In the oil gland section, the cavity of the left lobe is shown filled with secretion and the right lobe is shown empty. |
It is a bilobed holocrine structure. Each lobe contains a central cavity that collects the oily secretion and discharges through a duct. The lobes are separated entirely or partly, united at the apex in a projecting nipple.
Each of the lobes is autonomous, with its own supply of blood -vessels and nerves and can secrete independently of the other.
The excretory nipple, invested by the delicate skin, enclosed the efferent canals of the lobes and the terminal portions of their enveloped capsules.
The extremity may be naked furnished with a small tuft of down-like feathers called uropygial wick which may aid in transmitting oil from the gland to the beak while preening.
There may be strands of smooth muscle fibres circling the glandular orifice of well well-differentiated papilla.
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| Details of papilla types Comparative morphology and internal histology of the uropygial papilla. The figure explains the secretary apparatus. The central image shows the gland's relationship to the tail section and the feather tuft. The sub-figures (a,b, and c) illustrate the histological diversity of the papilla. |
Based on structure, three types of preen gland are known,
(a) Delicate type:
(b) Compact type:
(c) Unique passerine type:
With the two lobes coalesced, a strong reservoir, a glandular delicate nipple, without a terminal tuft of down, e.g. sparrow, blackbird and rook.
Differentiation between types of Papilla
(a) Simple (Delicate type)
Orifice: Single, narrow opening.
Wick System: Absent (direct contact).
Function: Slow, steady release.
Example: Pigeons, Songbirds.
(b) Tufted (Compact type)
Orifice: Broad, often multiple.
Wick System: Feather Tuft present.
Function: Rapid capillary "wicking".
Example: Ducks, Aquatic birds.
(c) Complex (Unique passerine type)
Orifice: Shielded or bifurcated.
Wick System: Internalised canal system.
Function: Regulated/Storage-heavy.
Example: Specialised Raptor species.
Physiology
The preen gland secretes rich oil of wax, fatty acids, fat and water, and is applied with the bill.
Preening:
It is a feather maintenance behaviour in which a bird tends to its feathers with its bill.
Procedure:
The bird grasps a feather near its base and then slides its bill along the shaft towards the tip of each feather, smoothing the barbs to re-position any broken connections and remove stale oil and dirt.
Birds may preen their feathers as often as once an hour while they are resting. They systematically rearrange and position out of positioned feathers especially the long flight feathers individually, and firmly restore the vane's integrity and remove parasites.
Physiology ( A Brief Note)
Proliferation: Basal cells at the periphery of the tubules divide rapidly.
Lipogenesis: Cells undergo a process of fatty degeneration to produce "preen oil". The cells move toward the centre, accumulating lipid droplets in their cytoplasm.
Disintegration: The entire cell breaks down (the "holocrine" method), releasing the lipid-rich "sebum" into the central lumen.
Excretion: Physical pressure from the bird's beak during preening "milks" the papilla (feather circlet), releasing the oil.
Chemical composition:
The secretion is just not an oil it is a complex mixture of;
- Monoester waxes (specifically fatty acids esterified with monohydric alcohols).
- Triglycerides and phospholipids.
- Odorous compounds (pheromones).
Function of the Uropygial Gland
It acts as a bacteriostat. Certain preen gland lipids protect against fungi and bacteria that digest keratin, so as to maintain insulation and colour.
To condition the feathers of the head, they can rub their head directly on the gland or the bird's head scratch to oil the feathers that it cannot reach with its bill.
Rub oil on its feet with its bill and then scratch.
( Studies say that birds that cannot reach their heads ( because of some deformities) to groom and delouse their heads and necks, tend to have more ectoparasites living on their heads. )
Others may promote the growth of nonpathogenic fungi and discourage feather lice. In hoopoes, musk duck, petrels and wood hoopoes of Africa, the preen gland secretion is foul-smelling which may also repel mammalian predators.
The uropygial gland is present in the majority of birds. The fact that it is usually large and well developed in aquatic birds has led to the belief that the oily secretion assists in rendering their feathers impermeable to water, in spite of much evidence to the contrary. For example in non non-aquatic bird like the oil bird (steatornis), the gland is equally large and well developed.
It is absent entirely in bustards.
The function of the gland is a topic of debate. The widespread belief that they are a 'dressing' for the plumage is supposed to be greatly exaggerated, as many birds without them succeed in keeping their feathers in as good condition as those equipped with them.
Recent work done view that that the oil gland secretion is a source of vitamin D taken down via mouth with feather particles by preening.
This relationship between the secretion of the gland and vitamin D is still not completely understood but the possibility of their presence may be related to the ecological presence or degree of presence of the antirachitic factor, as Friedmann has suggested.
Brief on the function of the Uropygial (preen) gland:
Feather Integrity: The secreted oil maintains the structural flexibility of the keratinous rachis and barbs, preventing the feathers from becoming brittle and breaking.
Hydrophobic Barrier: It gives a lipid-based coating aiding in water repellency, crucial for thermoregulation and buoyancy in aquatic species.
Antimicrobial & Antifungal Defence: Contains specific fatty acids that inhibit the growth of feather-degrading bacteria (like Bacillus licheniformis) and pathogenic fungi.
Ectoparasite Regulation: Acts as a barrier against certain lice and mites that feed on feather tissue.
Vitamin D Synthesis: In many species, the secretion contains 7-dehydrocholesterol, which is converted to Vitamin D3 upon exposure to UV light and then ingested during preening.
Pheromonal Communication: Serves as a source of "semiochemicals" that convey information about the bird’s sex, age, and reproductive status.
Short important practice Q&A
1. Based on the internal section, what is the primary role of the trabeculae?
View Answer
Correct Answer: Providing structural partitions for secretory tissue.
Rationale: Trabeculae are connective tissue septa that divide the gland into lobes and provide mechanical support for the secretory tubules.
2. Which classification describes the secretion method where the entire cell disintegrates?
View Answer
Correct Answer: Holocrine.
Rationale: In holocrine glands like the uropygial, cells accumulate lipids and then completely break down to release their contents.
3. In Figure 2, 'Type B' papilla is distinguished by which specific feature?
View Answer
Correct Answer: A specialized feather wick (tuft) at the orifice.
Rationale: The tuft facilitates rapid capillary wicking of oil, which is common in aquatic species needing efficient waterproofing.
4. What molecule in preen oil acts as a precursor for Vitamin D3?
View Answer
Correct Answer: 7-dehydrocholesterol.
Rationale: This sterol converts to Vitamin D3 upon exposure to UV light on the feathers and is then ingested during preening.
5. The primary ducts shown in the internal section function to:
View Answer
Correct Answer: Funnel oil from the lobes to the papilla.
Rationale: These ducts are the main transport conduits for the holocrine secretion to reach the exit orifice.
6. Why is the uropygial gland absent in birds like the Ostrich or certain Parrots?
View Answer
Correct Answer: They rely on "powder down" feathers.
Rationale: These specialized feathers crumble into a fine powder that provides similar protection and waterproofing to preen oil.
7. The "Preen Heart" shape seen in Diagram 1 is specifically caused by:
View Answer
Correct Answer: The bilobed anatomical structure of the gland.
Rationale: The two lobes meet at the midline above the tail, creating a characteristic heart-like silhouette in dorsal view.
8. How does preen oil provide a defence against ectoparasites like lice?
View Answer
Correct Answer: Specific fatty acids in the oil act as a natural chemical repellent.
Rationale: The chemical composition is deterrent to many mites and lice that would otherwise feed on the keratin of the feathers.
9. In detailed Diagram 2, which layer serves as the outermost boundary protecting the lobes?
View Answer
Correct Answer: The Capsule.
Rationale: The capsule is a tough layer of connective tissue that encloses the gland, providing both protection and structural integrity.
10. The complex duct system (Type C) is most advantageous for which group of birds?
View Answer
Correct Answer: Species requiring high-capacity storage and regulated release.
Rationale: Complex ducting allows for better control of the oil discharge, which is often seen in specialized raptors or high-performance flyers.
About the Author: This educational content on Zoology is written by Rekha Debnath, M.Sc. M.Phil. in Zoology, with a focus on university-level academic topics. Read the full Author Credentials and Background here.
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