preen gland, in birds, an organ located on the back near the base of the tail. Paired or in two united halves, it is found in most birds. Absent in ostrich, emu, cassowary, bustard, frogmouth, and a few other birds, the oil gland is best-developed in aquatic species, notably petrels and pelicans, and in the osprey and oilbird.

The secretions of the preen gland empty to the skin surface through one or more nipplelike pores. Most birds preen by rubbing their bill and head over the preen gland pore and then rubbing the accumulated oil over the feathers of the body and wings and the skin of the legs and feet. The oil is thought to help preserve the integrity of feather structure and, in some species, is also believed to be useful in preserving the horny structure of the bill and the scales of the legs and feet. It has also been speculated that, in at least some species, the oil contains a substance that is a precursor of vitamin D. This precursor substance is thought to be converted to vitamin D by the action of sunlight and then absorbed through the skin. Many ornithologists maintain that the function of the preen gland differs among various species of birds.