Closeup on a single white feather
A feather is one of the epidermal growths that forms the distinctive outer covering, or plumage, on a bird. They are the outstanding character that distinguishes the Class Aves from all others.
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.
It had been thought that feathers evolved from the scales of reptiles, however recent research casts doubt on this homology (for further reading see Q Rev Biol. 2002 Sep;77(3):261-95.). Feathers were most likely a filamentous insulation strucure, or possibly a marker for mating, and these later evolved to modern feathers.
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 the proto-birds Archaeopteryx and Microraptor.
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, providing an insulating layer. The vaned feathers include the remiges (flight-feathers), the rectrices (tail-feathers) and the contour feathers which are distributed over the whole body. A typical feather features a main shaft, called the rachis. Fused to the rachis are a series of branches, or barbs; the barbs themselves are also branched and form the barbules. These barbules have minute hooks for cross-attachment. At the base of the feather, the rachis expands to form the hollow tubular calamus, or quill, which inserts into a follicle in the skin.
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.
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.