one of the epidermal growths that form the distinctive outer covering, or plumage, on birds. Feathers are formed in tiny follicles in the epidermis, or outer skin layer, that produce keratin proteins. [ http://en.wikipedia.org/wiki/Feather ]
The beta-keratins in feathers, beaks and claws - and the claws, scales and shells of reptiles - are composed of protein strands hydrogen-bonded into beta-pleated sheets, which are then further twisted and crosslinked by disulfide bridges into structures even tougher than the alpha-keratins of mammalian hair, horns and hoof.
Among the characteristics that distinguish the extant Aves from other living groups. Feathers have also been noticed in those Theropoda which have been termed feathered dinosaurs. Although feathers cover most parts of the body of birds, they arise only from certain well-defined tracts on the skin. They aid in flight, thermal insulation, waterproofing and coloration that helps in communication and protection