How does a duck spread oil over it's feathers?

Ducks spread oil over their feathers using a special gland located near the base of their tail called the uropygial gland. When a duck preens its feathers, it uses its beak to extract the oil from this gland and then applies it to its feathers. This oil waterproofs their feathers, keeping them buoyant and insulated while also providing protection against dirt and parasites. Preening is an essential behavior for maintaining the health and functionality of their plumage.