Birds moult, or drop their feathers and grow new ones once a year or, in some kinds twice, for the same reason that you buy new suits of clothes. Feathers wear out. They get dirty and ragged from hard wear in all kinds of weather. Then they do not protect the bird's skin as well, keep him as healthy, or present as smooth a surface against the wind for flying. All animals moult, some only while growing, some at changing seasons. Fur-bearing animals, horses, cattle, dogs and cats grow thicker coats in winter and shed them in the spring. Snakes, crabs, lobsters and other animals shed their skins because they outgrow them. You shed your skin, but a little at a time, in tiny scales. You can see dead skin roll up when you rub yourself hard after a bath. You shed your hair, too. Old hairs fall out nearly every day, and new baby hairs grow in their places.