Birds' bodies are covered with an outer layer of fairly stiff but flexible contour feathers and an underlayer of fluffy down feathers. The contour feathers provide protection against wind, rain and snow while down feathers act as a layer of insulation. Tightly knit together and overlapping, feathers protect the skin and hold a layer of air over the bird's body. By adjusting their feathers, birds create and trap larger pockets of warm air near their skin, enhancing insulation. Most birds have an oil gland on their rump at base of the tail.
Secreted oil is rubbed over the feathers with the beak or bill.
This is known as preening. Preening conditions the feathers, creating a shield on that helps to block wind and repel water. Some birds, such as waterfowl, can survive in water that is close to freezing because the amount of oil in their feathers makes them waterproof. Waterfowl and other waterbirds also have a layer of fat that keeps them warm. Anyone who has ever gone outside on a cold, windy day without a hat knows that uncovered areas of the body lose heat.