Feather

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Feathers are

birds from other living groups.[4]

Although feathers cover most of the bird's body, they arise only from certain well-defined tracts on the skin. They aid in flight, thermal insulation, and waterproofing. In addition, coloration helps in communication and protection.[5] Plumology (or plumage science) is the name for the science that is associated with the study of feathers.[6][7]

Feathers have a number of utilitarian, cultural, and religious uses. Feathers are both soft and excellent at trapping

coats and sleeping bags. Goose and eider down have great loft, the ability to expand from a compressed, stored state to trap large amounts of compartmentalized, insulating air.[8] Feathers of large birds (most often geese) have been and are used to make quill pens. Historically, the hunting of birds for decorative and ornamental feathers has endangered some species and helped to contribute to the extinction of others.[9] Today, feathers used in fashion and in military headdresses and clothes are obtained as a waste product of poultry farming, including chickens, geese, turkeys, pheasants, and ostriches
. These feathers are dyed and manipulated to enhance their appearance, as poultry feathers are naturally often dull in appearance compared to the feathers of wild birds.

Etymology

Feather derives from the Old English "feþer", which is of Germanic origin; related to Dutch "veer" and German "Feder", from an Indo-European root shared by Sanskrit's "patra" meaning ‘wing’, Latin's "penna" meaning ‘feather’, and Greek's "pteron", "pterux" meaning ‘wing’.

Because of feathers being an integral part of quills, which were early pens used for writing, the word pen itself is derived from the Latin penna, meaning feather.[10] The French word plume can mean feather, quill, or pen.

Structures and characteristics

scanning electron microscopy
. Interlocking barbules are clearly seen in the middle image.

Feathers are among the most complex

hooves.[11][12] 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.[13]

Classification

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. The pennaceous feathers are vaned feathers. Also called contour feathers, pennaceous feathers arise from tracts and cover the entire body. A third rarer type of feather, the filoplume, is hairlike and are closely associated with pennaceous feathers and are often entirely hidden by them, with one or two filoplumes attached and sprouting from near the same point of the skin as each pennaceous feather, at least on a bird's head, neck and trunk.[14][15] Filoplumes are entirely absent in ratites.[16] In some passerines, filoplumes arise exposed beyond the pennaceous feathers on the neck.[1] The remiges, or flight feathers of the wing, and rectrices, or flight feathers of the tail, are the most important feathers for flight. A typical vaned 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 called barbicels for cross-attachment. Down feathers are fluffy because they lack barbicels, so the barbules float free of each other, allowing the down to trap air and provide excellent thermal insulation. 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. The basal part of the calamus is without vanes. This part is embedded within the skin follicle and has an opening at the base (proximal umbilicus) and a small opening on the side (distal umbilicus).[17]

Hatchling birds of some species have a special kind of natal down feathers (neossoptiles) which are pushed out when the normal feathers (teleoptiles) emerge.[1]

Flight feathers are stiffened so as to work against the air in the downstroke but yield in other directions. It has been observed that the orientation pattern of β-keratin fibers in the feathers of flying birds differs from that in flightless birds: the fibers are better aligned along the shaft axis direction towards the tip,[18][19] and the lateral walls of rachis region show structure of crossed fibers.[20][21]

Functions

Feathers insulate birds from water and cold temperatures. They may also be plucked to line the nest and provide insulation to the eggs and young. The individual feathers in the wings and tail play important roles in controlling flight.

predators for birds in their habitats, and serve as camouflage for predators looking for a meal. As with fish, the top and bottom colors may be different, in order to provide camouflage during flight. Striking differences in feather patterns and colors are part of the sexual dimorphism of many bird species and are particularly important in the selection of mating pairs. In some cases, there are differences in the UV reflectivity of feathers across sexes even though no differences in color are noted in the visible range.[22] The wing feathers of male club-winged manakins Machaeropterus deliciosus have special structures that are used to produce sounds by stridulation.[23]

Guinea fowl

Some birds have a supply of

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 in pennaceous feathers. They may be scattered in plumage as 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 powder down feathers and to spread them, while cockatoos may use their head as a powder puff to apply the powder.[24] Waterproofing can be lost by exposure to emulsifying agents due to human pollution. Feathers can then become waterlogged, causing the bird to 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 to reduce buoyancy, thereby allowing the birds to swim submerged.[25]

Rictal bristles of a white-cheeked barbet

Empidonax traillii) were found to catch insects equally well before and after removal of the rictal bristles.[27]

Grebes are peculiar in their habit of ingesting their own feathers and feeding them to their young. Observations on their diet of fish and the frequency of feather eating suggest that ingesting feathers, particularly down from their flanks, aids in forming easily ejectable pellets.[28]

Distribution

Contour feathers are not uniformly distributed on the skin of the bird except in some groups such as the

Coloration

Ramphocelus bresilius

The colors of feathers are produced by pigments, by microscopic structures that can refract, reflect, or scatter selected wavelengths of light, or by a combination of both.

Most feather pigments are

taxa – the yellow to red psittacofulvins[33] (found in some parrots) and the red turacin and green turacoverdin (porphyrin pigments found only in turacos
).

albinism in birds is caused by defective pigment production, though structural coloration will not be affected (as can be seen, for example, in blue-and-white budgerigars
).

The blues and bright greens of many parrots are produced by constructive interference of light reflecting from different layers of structures in feathers. In the case of green plumage, in addition to yellow, the specific feather structure involved is called by some the Dyck texture.[37][38] Melanin is often involved in the absorption of light; in combination with a yellow pigment, it produces a dull olive-green.

Emperor Pedro II of Brazil wearing a wide collar of orange toucan feathers around his shoulders and part of the Imperial Regalia
.

In some birds, feather colors may be created, or altered, by secretions from the uropygial gland, also called the preen gland. The yellow bill colors of many hornbills are produced by such secretions. It has been suggested that there are other color differences that may be visible only in the ultraviolet region,[24] but studies have failed to find evidence.[39] The oil secretion from the uropygial gland may also have an inhibitory effect on feather bacteria.[40]

The reds, orange and yellow colors of many feathers are caused by various carotenoids. Carotenoid-based pigments might be honest signals of fitness because they are derived from special diets and hence might be difficult to obtain,[41][42] and/or because carotenoids are required for immune function and hence sexual displays come at the expense of health.[43]

A bird's feathers undergo wear and tear and are replaced periodically during the bird's life through

molting. New feathers, known when developing as blood, or pin feathers, depending on the stage of growth, are formed through the same follicles from which the old ones were fledged. The presence of melanin in feathers increases their resistance to abrasion.[44] One study notes that melanin based feathers were observed to degrade more quickly under bacterial action, even compared to unpigmented feathers from the same species, than those unpigmented or with carotenoid pigments.[45] However, another study the same year compared the action of bacteria on pigmentations of two song sparrow species and observed that the darker pigmented feathers were more resistant; the authors cited other research also published in 2004 that stated increased melanin provided greater resistance. They observed that the greater resistance of the darker birds confirmed Gloger's rule.[46]

Although sexual selection plays a major role in the development of feathers, in particular, the color of the feathers it is not the only conclusion available. New studies are suggesting that the unique feathers of birds are also a large influence on many important aspects of avian behavior, such as the height at which different species build their nests. Since females are the prime caregivers, evolution has helped select females to display duller colors down so that they may blend into the nesting environment. The position of the nest and whether it has a greater chance of being under predation has exerted constraints on female birds' plumage.[47] A species of bird that nests on the ground, rather than the canopy of the trees, will need to have much duller colors in order not to attract attention to the nest. The height study found that birds that nest in the canopies of trees often have many more predator attacks due to the brighter color of feathers that the female displays.[47] Another influence of evolution that could play a part in why feathers of birds are so colorful and display so many patterns could be due to that birds developed their bright colors from the vegetation and flowers that thrive around them. Birds develop their bright colors from living around certain colors. Most bird species often blend into their environment, due to some degree of camouflage, so if the species habitat is full of colors and patterns, the species would eventually evolve to blend in to avoid being eaten. Birds' feathers show a large range of colors, even exceeding the variety of many plants, leaf, and flower colors.[48]

Parasites

The feather surface is the home for some ectoparasites, notably feather lice (

phoresy. This life history has resulted in most of the parasite species being specific to the host and coevolving with the host, making them of interest in phylogenetic studies.[49]

Feather holes are chewing traces of lice (most probably Brueelia spp. lice) on the wing and tail feathers. They were described on barn swallows
, and because of easy countability, many evolutionary, ecological, and behavioral publications use them to quantify the intensity of infestation.

Parasitic cuckoos which grow up in the nests of other species also have host-specific feather lice and these seem to be transmitted only after the young cuckoos leave the host nest.[50]

Birds maintain their feather condition by

anting, in which ants are introduced into the plumage, helps to reduce parasites, but no supporting evidence has been found.[51]

Human usage

Utilitarian

Bird feathers have long been used for

arrows. Colorful feathers such as those belonging to pheasants have been used to decorate fishing lures
.

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.[52] Feathers may also be useful in the non-destructive sampling of pollutants.[53]

The poultry industry produces a large amount of feathers as waste, which, like other forms of keratin, are slow to decompose. Feather waste has been used in a number of industrial applications as a medium for culturing microbes,[54] biodegradable polymers,[55] and production of enzymes.[56] Feather proteins have been tried as an adhesive for wood board.[57]

Some groups of Native people in Alaska have used ptarmigan feathers as temper (non-plastic additives) in pottery manufacture since the first millennium BC in order to promote thermal shock resistance and strength.[58]

In religion and culture

Mexican featherwork painting of Isidore the Laborer made from duck, hummingbird, and canary feathers. This style of painting, popular during the Novohispanic era, integrates featherwork of pre-Hispanic origin with Christian iconography. 18th century, Museo Soumaya

Eagle feathers have great cultural and spiritual value to American Indians in the US and First Nations peoples in Canada as religious objects. In the United States, the religious use of eagle and hawk feathers is governed by the eagle feather law, a federal law limiting the possession of eagle feathers to certified and enrolled members of federally recognized Native American tribes.

In South America, brews made from the feathers of

Indian peacock have been used in traditional medicine for snakebite, infertility, and coughs.[60][61]

Members of Scotland's Clan Campbell are known to wear feathers on their bonnets to signify authority within the clan. Clan chiefs wear three, chieftains wear two and an armiger wears one. Any member of the clan who does not meet the criteria is not authorized to wear feathers as part of traditional garb and doing so is considered presumptuous.[62]

During the 18th, 19th, and early 20th centuries, there was a booming international trade in plumes for extravagant women's hats and other headgear (including in

Lacey Act in 1900, and to changes in fashion. The ornamental feather market then largely collapsed.[64][65]

More recently, rooster plumage has become a popular trend as a hairstyle accessory, with feathers formerly used as fishing lures now being used to provide color and style to hair.[66]

Feather products manufacturing in Europe has declined in the last 60 years, mainly due to competition from Asia. Feathers have adorned hats at many prestigious events such as weddings and Ladies Day at racecourses (Royal Ascot).

Evolution

Rachis-dominated feathers inside mid-Cretaceous Burmese amber

Functional considerations

The functional view on the evolution of feathers has traditionally focused on insulation, flight and display. Discoveries of non-flying Late Cretaceous feathered dinosaurs in China,

Tupandactylus imperator, the features are so well preserved that the melanosome (pigment cells) structure can be observed. By comparing the shape of the fossil melanosomes to melanosomes from extant birds, the color and pattern of the feathers on Anchiornis and Tupandactylus could be determined.[75][76] Anchiornis was found to have black-and-white-patterned feathers on the forelimbs and hindlimbs, with a reddish-brown crest. This pattern is similar to the coloration of many extant bird species, which use plumage coloration for display and communication, including sexual selection and camouflage. It is likely that non-avian dinosaur species utilized plumage patterns for similar functions as modern birds before the origin of flight. In many cases, the physiological condition of the birds (especially males) is indicated by the quality of their feathers, and this is used (by the females) in mate choice.[77][78] Additionally, when comparing different Ornithomimus edmontonicus specimens, older individuals were found to have a pennibrachium (a wing-like structure consisting of elongate feathers), while younger ones did not. This suggests that the pennibrachium was a secondary sex characteristic and likely had a sexual function.[79]

Molecular evolution

Several genes have been found to determine feather development. They will be key to understand the evolution of feathers. For instance, some genes convert scales into feathers or feather-like structures when expressed or induced in bird feet, such as the scale-feather converters Sox2, Zic1, Grem1, Spry2, and Sox18.[80]

Feathers and scales are made up of two distinct forms of

crocodilians indicates that it was inherited from a common ancestor.[81]

This may suggest that crocodilian scales, bird and dinosaur feathers, and pterosaur

pycnofibres are all developmental expressions of the same primitive archosaur skin structures; suggesting that feathers and pycnofibers could be homologous.[82] Molecular dating methods in 2011 show that the subfamily of feather β-keratins found in extant birds started to diverge 143 million years ago, suggesting the pennaceous feathers of Anchiornis were not made of the feather β-keratins present in extant birds.[83] In 2019, scientists found that genes for the production of feathers evolved at the base of archosauria, supporting that feathers were present at early ornithodirans and is consistent with the fossil record.[84]

Feathered dinosaurs

Archaeopteryx lithographica
(Berlin specimen)