Feathers are one of the epidermal growths that form the distinctive plumage on birds.
Feathers are among the most complex structural organs found in vertebrates: integumentary appendages, formed by controlled proliferation of cells in the epidermis, or outer skin layer, that produce keratin proteins. The Î²-keratins in feathers, beaks and clawsâ€”and the claws, scales and shells of reptilesâ€”are composed of protein strands hydrogen-bonded into Î²-pleated sheets, which are then further twisted and crosslinked by disulfide bridges into structures even tougher than the Î±-keratins of mammalian hair, horns and hoof.
Feathers insulate birds from water and cold. The individual feathers in the wings and tail play important roles in controlling flight. These have their own identity and are not just randomly distributed. Some species have a crest of feathers on their heads. Although feathers are light, a birdâ€™s plumage weighs two or three times more than its skeleton, since many bones are hollow and contain air sacs. Color patterns serve as camouflage against predators. As with fish, the top and bottom colors may be different to provide camouflage during flight. The remarkable colors and feather sizes of some species have never been fully explained.
There are two basic types of feather: vaned feathers which cover the exterior of the body, and down feathers which are underneath the vaned feathers. The pennaceous feathers are vaned feathers. Also called contour feathers, pennaceous feathers are distributed over the whole body. Some of them are modified into remiges, the flight feathers of the wing, and rectrices, the flight feathers of the tail. Down feathers are fluffy because they lack barbicels, so the barbules float free of each other, allowing the down to trap much air and provide excellent thermal insulation.