Preening is the art, practiced by animals, of cleaning, grooming, and maintaining parts of the body.

Individual animals regularly clean themselves and put their fur, feathers or other skin coverings in good order. This activity is known as personal grooming, a form of hygiene. Extracting foreign objects such as insects, leaves, dirt, twigs and parasites, are all forms of grooming. Among animals, birds spend considerable time preening their feathers. This is done to remove ectoparasites, keep them in good aerodynamic condition, and waterproof them. To do that, they use the preen oil secreted by the uropygial gland, the dust of down feathers, or other means such as dust-bathing or anting. During oil spills, animal conservationists that rescue penguins sometimes dress them in knitted sweaters to stop them from preening and thereby ingesting the mineral oil which is poisonous. Monkeys may also pick out nits from their fur or scratch their rears to keep themselves clean. Felidae cats are well known for their extensive grooming. One reason advanced for such grooming is to remove all traces of blood and other matter so as to not alert prey with the scent. Cats groom so much that they often produce hairballs from the fur they ingest.

The uropygial gland, informally known as the preen gland or the oil gland, is a gland possessed by the majority of birds. It is located dorsally at the base of the tail and is greatly variable in both shape and size.

The uropygial gland is strongly developed in many waterbirds, such as ducks, petrels, pelicans and in the osprey and the oilbird. It appears that the waterproofing effect is not primarily by the uropygiols – although they are hydrophobic – but by applying an electrostatic charge to the oiled feather through the mechanical action of preening. A study examining the gland's mass relative to body weight in 126 bird species showed the absence of a clear cut correlation between the gland's mass and the degree of birds' contact with water.

The preen oil is believed to help maintain the integrity of the feather structure. In waterbirds, the oil maintains the flexibility of feathers and keeps feather barbules from breaking. The interlocking barbules, when in good condition, form a barrier that helps repel water (see below). In some species, preen oil is also believed to maintain the integrity of the horny beak and the scaly skin of the legs and feet. It has also been speculated that in some species, the oil contains a precursor of vitamin D; this precursor is converted to vitamin D by the action of sunlight and then absorbed through the skin.

See also:
Personal Grooming
Uropygial Gland