The uropygial (\ˌyu̇r-ə-ˈpī-jē-əl-\) gland, usually simply called the oil gland, or the preen gland, is a small gland found in almost all birds that secretes an oil that is used for preening. The small nub is found near the base of the tail. It is usually covered by feathers and hard, if not impossible, to see when the bird isn't preening. The chief compounds of preen oil are diester waxes called uropygiols. The oil is used to keep the feathers clean and dry.
Chickens typically transfer the oil from this gland to their feathers by rubbing their head onto the gland, which has a tuft of specialized feathers to serve as a kind of brush , and then rub the oil into their feathers for weatherproofing.
Chickens do not have uropygial glands that are as developed as waterfowls', still the glands do have a waterproofing effect on their feathers. This can be seen when a drop of water is dropped onto a chicken's back: it will simply roll off. While coated with preen oil, feathers are not greasy at all to the touch but rather smooth and shiny. Araucanas are alone in the poultry kingdom, for not having an uropygial gland.