Yesterday’s unusual-looking structure was, as the feather quills next to it may have indicated to you, located on the body of a bird, specifically, a Barred Owl. Usually hidden by feathers, this oil gland, known as a uropygial or preen gland, is found just above the base of a bird’s tail and contains fatty acids, fat and wax. The shape of the gland varies greatly among species. Its nipple-like projection (papilla) can have between one and five openings and in many species is covered by a tuft of specialized feathers.
The uropygial gland plays an important part in feather conditioning. Birds preen, or groom, their feathers with their bill in order to keep their feathers in good condition. The act of preening removes dust, dirt and parasites from feathers and aligns each feather in the optimum position relative to adjacent feathers and body shape. Preening is also the means by which oil from the uropygial gland is applied to a bird’s feathers, feet and legs. The act of preening induces the flow of the secretion from the nipple of the uropygial gland. By manipulating the gland with its bill and rubbing it with its head, a bird accumulates oil and then transfers this oil from its bill and head to the rest of its body.
Ornithologists theorize that the uropygial gland serves different purposes in different species of birds. While the secretions contain beneficial bacteria, and keep a bird’s bill, legs and feet in good condition, waterproofing is the primary function of this gland (which is especially well developed in water birds). Both the water-repellent secretions as well as the act of applying them to feathers through preening serve to protect a bird’s body from water and the resulting heat and flight loss. Some birds lack uropygial glands but have specialized feathers that disintegrate into powder down, which serves the same purpose as preen oil. (Photo: Anhinga preening with uropygial gland secretions; thanks to Joan Waltermire for yesterday’s Barred Owl uropygial gland photo op)