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.
Hatchling birds of some species have a special kind of natal down (neossoptiles) and these are pushed out when the normal feathers (teleoptiles) emerge.
Flight feathers are stiffened so as to work against the wind in the downstroke but yield in other directions. It is noted that the pattern of orientation of β-keratin fibers in the feathers of flying birds differs from that in flightless birds. The fibers are better aligned in the middle of the feather and less aligned towards the tips.
Some birds have a supply of powder down feathers which 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. Powder down has evolved independently in several taxa and can be found in down as well as pennaceous feathers. They may be scattered in plumage in the pigeons and parrots or in localized patches on the breast, belly or flanks as in herons and frogmouths. Herons use their bill to break the feathers and to spread them while cockatoos may use their head as a powder puff to apply the powder. Waterproofing can be lost by exposure to emulsifying agents due to human pollution. Feathers can become waterlogged and birds may sink. It is also very difficult to clean and rescue birds whose feathers have been fouled by oil spills. The feathers of cormorants soak up water and help in reducing buoyancy and thereby allowing the birds to swim submerged.
Bristles are stiff, tapering feathers with a large rachis but few barbs. Rictal bristles are bristles found around the eyes and bill. They may serve a similar purpose to eyelashes and vibrissae in mammals. It has been suggested that they may aid insectivorous birds in prey capture or that it may have sensory functions, however there is no clear evidence. In one study, Willow Flycatchers (Empidonax traillii) and they were found to catch insects equally well before and after removal of the rictal bristles.
Grebes are peculiar in their habit of ingesting their own feathers and also feeding them to their young. Observations on the diet and feather eating frequency suggest that ingesting feathers particularly down from their flanks aids in forming easily ejectable pellets along with their diet of fish.
Contour feathers are not uniformly distributed on the skin of the bird except in some groups such as the Penguins, ratites and screamers. In most birds the feathers grow from specific tracts of skin called pterylae while there are regions which are free of feathers called apterylae. Filoplumes and down may arise from the apteriae, regions between the pterylae. The arrangement of these feather tracts, pterylosis or pterylography, varies across bird families and has been used in the past as a means for determining the evolutionary relationships of bird families.
The blues and greens of many parrots are produced by constructive interference of light reflecting from different layers of the structures in feathers in addition to the yellow carotenoid pigments. Melanin is often involved in the absorption of some of the light in these feathers. The specific feather structure involved is sometimes called the Dyck texture.
Albinism is caused by the lack of pigment in some or all of a bird's feathers.
In some birds, the feather colors may be created or altered by uropygial gland secretions. The yellow bill colors of many hornbills are produced by preen gland secretions. Other differences that may only be visible in the ultraviolet region have been suggested but studies have failed to find evidence. Uropygial oil secretion may also have an inhibitory effect on feather bacteria.
A bird's feathers undergo wear and tear and are replaced periodically during its life through molting. New feathers are formed through the same follicle from which the old ones were fledged. The presence of melanin in feathers increases their resistance to abrasion. Melanin based feathers were however found to be faster degraded by bacteria than those with carotenoid pigments. This has led to the suggestion that Gloger's rule, the observation that birds from more humid regions tend to be darker may be related to the increased bacterial load and the selection for greater melanin. The evolution of coloration is based on sexual selection and it has been suggested that carotenoid based pigments may have evolved since they are likely to be more honest signals of fitness since they are derived from special diets.
Birds maintain their feather condition by bathing in water, dust bathing and preening. A peculiar behaviour of birds, anting, where ants are introduced into the plumage was suggested to help in reducing parasites but no supporting evidence has been found.
Feathers have a number of utilitarian and cultural and religious uses.
During the 18th, 19th, and even 20th Centuries a booming international trade in plumes, to satisfy market demand in North America and Europe for extravagant head-dresses as adornment for fashionable women, caused so much destruction (for example, to egret breeding colonies) that a major campaign against it by conservationists led to the Lacey Act and caused the fashion to change and the market to finally collapse. Frank Chapman noted in 1886 that as many as 40 species of birds were used in about three-fourths of the 700 ladies' hats that he observed in New York City.
Feathers of large birds (most often geese) have been and are used to make quill pens. The word pen itself is derived from the Latin penna for feather. The French nom-de-plume for pen name has a similar origin.
Feathers are also valuable in aiding the identification of species in forensic studies, particularly in bird strikes to aircraft. The ratios of Hydrogen isotopes in feathers help in determining the geographic origins of birds. Feathers may also be useful in the non-destructive sampling of pollutants.
The poultry industry produces a large amount of feathers as waste and like other forms of keratin are slow in their decomposition. Feather waste has been used in a number of industrial applications as a medium for culturing microbes, biodegradeable polymers, and production of enzymes. Feather proteins have been tried as an adhesive for wood board.
Various birds and their plumages serve as cultural icons throughout the world, from the hawk in ancient Egypt to the bald eagle and the turkey in the United States. In Greek mythology, Daedelus the inventor and Icarus tried to escape his prison by attaching feathered wings to his shoulders with wax, which was melted by the Sun.
In South America, brews made from the feathers of Condors are used in traditional medications. In India, feathers of the Indian Peacock have been used in traditional medicine for snakebite, infertility and coughs.