Birds preen themselves to clean and waterproof their feathers, to maintain their general health and to keep them lying smooth and neat. This preening or grooming starts as the nestling's feathers are breaking out of their sheaths. The young bird spends a great deal of time combing the feathers with its bill and freeing them from bits of sheath and other blemishes.
In adult life the bird continues this behaviour and also uses the preen glands or oil glands which are located on the back, just in front of the tail. The birds nibble at the glands and rub their heads against them, spreading the secreation on their feathers. This oil waterproofs the plumage, keeps it supple and maintains its insulating qualities. The oil may also be useful as a source of Vitamin D if swallowed accidentally by the bird.
Many kinds of birds have preen glands. They often combine preening with dust or water bathing. Ducks may be seen dipping their heads under water, flicking their wings and wriggling their bodies while preening. This seems to give them great pleasure and fun.