Bird feathers, which are composed of keratin, are shed and regrown through a process known as moult. This natural cycle allows feathers to change throughout a bird's life, indicating its sex, maturity, and the current season.
Early feathered animals, including bird-sized dinosaurs, initially used feathers for warmth and display rather than flight. The evolution of feathers to create a smooth, zipping surface later enabled flight.
The appearance of feathers can signify different seasons, such as the fuzzy down of a late-hatched moorhen chick representing summer, or the white heads of gulls indicating winter. A mallard's changing plumage can also herald the coming spring.