Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They are 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.
Structures and characteristics
Feathers are among the most complex integumentary appendages found in vertebrates and are formed in tiny follicles in the epidermis, or outer skin layer, that produce keratin proteins. The β-keratins in feathers, beaks and claws — and the claws, scales and shells of reptiles — are composed of protein strands hydrogen-bonded into β-pleated sheets, which are then further twisted and crosslinked by disulfide bridges into structures even tougher than the α-keratins of mammalian hair, horns and hoof. The exact signals that induce the growth of feathers on the skin are not known, but it has been found that the transcription factor cDermo-1 induces the growth of feathers on skin and scales on the leg.
All birds have feathers and only birds have feathers even if the feathers are highly modified as on penguins.
While most feathers share a common overall structure, there are several different kinds of feathers adopted for specialized roles. Changes in feather structure provide the adaptations necessary for feathers to be used in many different ways.
Feathers also support the behavior of the bird within its environment and its lifestyle. Feathers that support the soaring flight of an eagle have a much different role than the feathers that protect an American Dipper, which spends much of its time in fast-flowing streams.