Birds preen their feathers to keep them in prime condition. Preening improves the look of the feathers, removes dust and parasites, and aligns the feathers.
As a bird preens, it secretes oil from the uropygial gland, which is spread to every feather and helps to waterproof the feathers. Aligning the feathers makes them aerodynamic for better flight.
A bird uses its beak and feet to preen, but there are many other methods involved with preening. Water and dust baths help remove dust and parasites. Stretching provides more room to align feathers. Birds also preen each other as a courtship ritual.