The Preen Gland
The uropygial gland is a bilobed holocrinc gland. It is the principle cutaneous gland of birds. It is present in most species of bird, and it is relatively large in some aquatic species. Among the psittacine species that do not possess a uropygial gland are the hyacinth macaw, Anodorhynchus hyacinthinus , the Lear's macaw, Anodorhynchus leari, and the Spix's macaw, Cyanopsitta spixii. All of the parrots in the genus Amazona also do not posses a uropygial gland. The gland is present in the other psittacine species.
The uropygial gland, when present, lies on the mid-line dorsally on the trunk in the rump area above the levator muscles of the pygostyle.There are no feathers normally on the skin over the gland. There is, however, a tuft of down feathers at the tip of the papilla in most species, and this is called the uropygial wick. In many species of bird, the tail usually flexes laterally each time the bird reaches around to contact the gland and the wick.2
The gland secretion is complex and consists of a combination of extruded cells, ester waxes, fatty acids, fat and sudanophilic secretory granules.1,2 The secretion is spread over the feathers during the act of preening. Waterproofing is considered to be one function of the secretion (although it is not necessary for it), and another function is the suppression of the growth of organisms on the skin. The secretion helps keep the feathers, beak, and scales supple.
The secretion from the uropygial gland also contains vitamin D precursors that are also spread over the feathers by preening. With exposure to the ultraviolet portion of sunlight, the secretions are converted to an active form, vitamin D3, which is then ingested with subsequent preening.3
New research shows that the bird eye sees light in the ultraviolet range, and the secretion from the uropygial gland may also play a role in the identification of the sex of a bird, and may be involved with individual identification of birds, as well.4 In primates, the lens acts as a yellow filter which cuts off light of wavelengths below 400nm and therefore renders ultraviolet radiation invisible. However in diurnal birds, the cornea and lens are optically clear and appear to transmit wavelengths down to about 350nm, thus rendering near ultraviolet radiation visible.1 The lens only absorbs those ultraviolet wavelengths that are not physiologically destructive.1
References
1. King AS, McLelland J. Birds, their structure and function. Bailliere Tindall, London, 1984:28,290.
2. Evans HE, Anatomy of the budgerigar. In:Petrak ML (cd). Diseases of cage and aviary birds. Second ed. Philadelphia: Lea and Febiger, 1982:127,613.
3. Cooper JE, Harrison GJ. Dermatology. In:Ritchie BW, Harrison GJ, Harrison LR (cds) Avian Medicine: principles and application. Lake Worth, FL. Wingers Publishing Inc, 1994:613-614.
4. Pepperberg I. Discussion during keynote address. Speaker unidentified.
From: ExoticPetVet.com