any of a variety of skin structures that secrete oily or greasy substances of various functions. In birds, the preen gland, or uropygial gland, ... Found on http://www.britannica.com/eb/a-z/o/10
A small organ located above the tail at the base of the rump on most birds. Also called a uropygial gland or preen gland. Birds used their bill to press oil from the gland and then spread it over their feathers for insulation, waterproofing, feather pliability and health. Found on http://www.sialis.org/glossary.htm
A gland, located at the base of the tail on the dorsal side of the bird's body, that secretes oils that birds spread over their feathers during preening. The oils keep the skin supple and the feathers and scales from becoming brittle, but they do not appear to waterproof the feathers. Also called the uropygial gland or preen gland. Found on https://academy.allaboutbirds.org/bird-academys-a-to-z-glossary-of-bird-ter