Feather
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A feather is one of the epidermal growths that forms the distinctive outer covering, or plumage, on a bird. They are the outstanding characteristic that distinguishes the Class Aves from all other living groups. Other Theropoda also had feathers (see Feathered dinosaurs).
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Characteristics
Feathers are the most complicated integumentary structure among the vertebrates. Like hair, nails and scales, feathers are integumentary appendages; skin organs that form by controlled proliferation of cells in the epidermis, or outer skin layer, that produce keratin proteins. They insulate birds from water and cold temperatures and provide colour which is sometimes used as camouflage against predators and sometimes as a means of visual communication. Although individual feathers are very light, a bird's plumage weighs two or three times more than its skeleton.
A bird's feathers are replaced periodically during its life through molting, new feathers are formed through the same follicle from which the old ones were fledged.
Some birds have a supply of powder down. Powder down feathers grow continuously, with small particles regularly breaking off from the ends of the barbules. These particles produce a powder that sifts through the feathers on the bird's body and acts as a waterproofing agent and a feather conditioner. Most waterbirds produce a large amount of powder down.
Bristles are stiff, tapering feathers with a large rachis but few barbs. Rictal bristles are bristles found around the eyes and bill. They serve a similar purpose to eyelashes and vibrissae in mammals.
Origins
Feathers most likely originated as a filamentous insulation structure, or possibly as markers for mating, with flight emerging only as a secondary purpose. It had been thought that feathers evolved from the scales of reptiles, but recent research casts doubt on this homology (see Quarterly Review of Biology 77:3 (September 2002): 261-95). Experiments show that the same protein (when missing before birth) that causes bird feet to stay webbed, causes reptile scales to become feathers. [[1] (http://www.dinosauria.com/jdp/archie/scutes.htm)
Feathered dinosaurs
Main article: Feathered dinosaurs
Although birds use feathers primarily for flight, several dinosaurs have been discovered with feathers on their limbs that would not have functioned for flight. One theory is that feathers originally developed on dinosaurs as a means of insulation; those small dinosaurs that then grew longer feathers may have found them helpful in gliding, which would have begun the evolutionary process that resulted in some proto-birds like Archaeopteryx and Microraptor zhaoianus. Other dinosaurs discovered with feathers include Pedopenna daohugouensis, Sinosauropteryx, and Dilong paradoxus. Currently the question is whether birds are deinonychosaurians or dromaeosaurids, not whether birds are dinosaurs. It has been suggested that Pedopenna is older than Archaeopteryx, however, their age remains doubted by some experts. Dilong is a tyrannosauroid which predates Tyrannosaurus rex by 60 to 70 million years.
Human uses
Feathers are both soft and excellent at trapping heat; thus, they are sometimes used in high-class bedding, especially pillows, blankets, and mattresses. They are also used as filling for winter clothing, such as coats. They have also been put to use as sexual aids; see feather dancing. Colorful feathers such as those belonging to pheasants have been used in the past to decorate hats and fishing lures. In Greek mythology, Icarus tried to escape his prison by attaching feathered wings to his shoulders. Feathers can also be used to tickle people.