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Birds spend considerable time cleaning and arranging 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.
Where preen oil is used the bird stimulates the production of oil with its beak and spreads it over the feathers with a stroking and quivering movement.