Found only on birds, feathers are some of the most complex integumentary (skin) structures found in vertebrates.
There are two primary types of feathers: flight and down. Some birds, such as peacocks and birds-of-paradise, also have display feathers.
Flight feathers come in two categories of their own: remiges (wing feathers), and rectrices (tail feathers). These strong feathers are what allow most birds to fly. They have strong midline ridges, called the shaft (or rachus). The rachus is mostly covered in the fluffy-but-structured extensions, what most people think of as the “feather”, called the vane. At the bottom of the rachus, below the vane, is the bare quill (or calamus). Each extension on the vane is covered in many hooks and hooklets (also called barbs and barbicels), which is what birds are putting back into place when they preen - gotta keep everything in order to fly well!
Down feathers don’t have the hooks or the rachus of the flight feathers. There are many types of down feathers, but all of them serve the same basic purpose, which is temperature regulation. They trap air close to the bird’s body, which insulates it from the cold. When a bird puffs up in the middle of winter, it’s creating a bigger insulating layer to trap body heat, so that it doesn’t cool off as quickly.
To replace their feathers on a regular basis (to remove damaged or correct for lost feathers), birds undergo a process called moulting. Depending upon the species, this can happen all at once, be staggered over the course of a year, or happen gradually over several years. Many species undergo moulting during breeding season, to show off their flashy plumage.
Moulting is regulated by the pineal gland and circadian rhythms in most birds, though in pet birds, a somewhat-constant slow moult (punctuated by one or two “big moults” a year) is found, as a result of artificial lighting.
Nouveau Larousse Illustre. Claude Auge, 1898.
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