Some swans have more than 25,000. Birds are the only animals with feathers. When you look at a perched bird, most of the feathers you see are small ones that give its body a warm, smooth covering. When flying, birds show the larger, stiffer wing feathers used for flight. A single feather may have more than one million parts.
Feathers become worn with use and are usually replaced every year.
This is called molting and is generally gradual, enabling birds to fly reasonably well. Ducks and geese molt all their wing feathers at once in the autumn and are unable to fly for two to four weeks. Birds are the only animals that have hollow bones in order to keep their body weight low to assist in flying. The color of a bird s plumage can play a part in its search for food. Terns and many other fish-eating sea birds have white underparts.