A Guide To Bird Feathers

bird-feathers
King of Saxony Bird of Paradise, by Nick Athanas.

Bird Feathers

Bird feathers are one of the most distinctive features of avian anatomy. Feathers are fundamental to many aspects of a bird’s existence. They provide insulation essential for controlling body temperature, aerodynamic power necessary for flight, colors used for communications, and camouflage. Feathers in their standard or specialized forms are used for a variety of other roles such as sound production, as nesting material, hearing, cleanliness, parasite repellent, water transport, tactile sensation, organ protection, and courtship display. Bird feathers are important in aiding with bird identification.

Table of Content

Composition of a Bird Feather

All feathers are composed of keratin, an inactive component resistant to external agents such as solar radiation, water, and protein-digesting enzymes. Structurally, feathers are composed of microscopic filaments compacted or embedded in a keratin matrix. These elements make feathers last for a long time as a functional part of a bird’s body.

 

bird-feathers
Wing and tail feathers (left) and a microscopic view of the rachis, barbs, and barbules. Photo by Andrew_PSm.

Feather Structure

A typical feather’s primary structure is a long, central shaft and a broad flat vane on either side of the shaft. The shaft that supports the vanes is called the rachis. The bottom part of the rachis is called the calamus, which is hollow, supports no vanes, and anchors the feather in a follicle below the bird’s skin’s surface.

The vanes are composed of barbs that come off the rachis, tightly interlocked by barbules resulting in a cohesive flat fabric-like structure.

Said it differently, the rachis’ lateral branches are called barbs and constitute the vanes’ primary elements. Each barb consists of a tapered central axis, the ramus, with rows of smaller barbules projecting from both sides of a barb. Barbs and barbules form an interlocking but flexible surface.

Feather Modifications

Bird feathers are subject to striking modifications. For instance, an extreme fusion of the barbs results in feathers that look like strips of plastic like the central tail feather of Wilson’s Bird of Paradise (Cicinnurus respublica). Modified feathers are typically used in courtship displays. The rachis also varies from thin and flexible to rigid. The Marvelous Spatuletail (Lodigesia mirabilis) has two long and thin rachis that ends on a vaned flag at the tip. Woodpecker and woodcreepers have stiff bracing tail feathers for support. Feather and plumage aberrations such as leucism, albinism, and xanthochroism or xanthism are not considered feather modifications.

Types of Feathers

The most conspicuous feathers are vaned feathers, which include contour feathers covering the body surface and the larger flight feathers of the wing and tail. The smooth overlapping arrangement of the vaned feathers reduces air turbulence in flight.

Primary and secondary flight feathers.

Flight Feathers

The flight feathers are long, broad, stiff, and almost entirely pennaceous feathers without an afterfeather. A pennaceous feather has a shaft with a basal part, called a calamus, and is embedded in the skin. Flight feathers are adapted, primarily, to aerodynamic functions and have very little importance in insulation. The flight feathers of the wing are called remiges and are composed of primary remiges and secondary remiges: Combined, these feathers from the main horizontal surface of a wing. Rows of smaller feather called coverts overlap de basis of the remiges and cover the gaps between them.

Primary Flight Feathers

The long shafts of the primary flight feather or primary remiges attach to the bird’s hand bones. These feathers provide the forward thrust on the downstroke of the wing during flight.

Primary remiges have asymmetrical vane areas showing a narrow and stiffer outer vane on the leading edge and a wider and somewhat softer vane on the trailing side.

Most birds have ten primary remiges, but this number varies to nine in some passerine birds, eleven in grebes, storks, flamingos, and sixteen in ostriches.

Specialized barbules, called friction barbules, found in the inner vanes of the outer primary feathers, reduce slippages and separation of the feathers during flight.

Modification of Primary Remiges

Because flight efficiency is directly linked to the primary remiges’ structure, major structural modifications are uncommon. However, a few exceptions of primary flight feather modification include the sickle-winged guan, blue-throated piping guan, and male woodcock. These birds use the modified primary remiges to produce sound used during courtship display. An extreme case of a modification of the primary flight feathers is that of the standard winged nightjar. This crepuscular bird has long projections of a primary feather with vanes at the end the bird uses for courtship display.

Secondary Flight Feathers

The inner or secondary flight feathers attach to the ulna and form much of the inner wing surface. These feathers generate lift during flight. The secondary flight feathers range from six in hummingbirds to 19 in the great horned owl and 40 in albatrosses.

modified-bird-feathers
Mandarin Duck with modified head, neck, and secondary wing feathers.

Modification of Secondary Flight Feathers

In some species, such as the mandarin duck’s flag-like feather, the secondary flight feathers have been modified for courtship display purposes. The secondary flight feathers have been thickened in the club-winged manakin (Machaeropterus deliciosus), which produces a mechanic sound when the bird claps its secondary flight feathers.

Tail Feathers

The tail feathers or rectrices attach to the fused caudal vertebrae or pygostyle. There are usually 12 rectrices that function primarily in steering and braking during flight. As with other flight feathers, the rectrices have long rachis and, generally, vanes of equal length on both sides of the rachis.

Modification of Tail Feathers

In some birds, the tail feathers have developed ornate lengths and shapes primarily for courtship display, though they can be a handicap during flight. Birds with elaborate tail feathers include pheasants, lyrebirds, birds of paradise, and perhaps the most conspicuous of them all the peacock. Male marvelous spatuletails (Lodigesia mirabilis) have only four rectrices, two of which are only thin, flexible rachis about 15 cm long with a large flag or vaned tip.

Tail feathers are also modified for sound production in some snipes, hummingbirds, and bracing support, such as woodpeckers, woodcreepers, and swifts.

types-feathers 

Contour Feathers

The contour or body feathers are generally composed of a fluffy basal portion, which remains hidden, and a cohesive longer portion above the fluffy part of the feather. The more extended portion of the feather constitutes the visible plumage of a bird.  These feathers are arranged in an overlapping pattern like shingles on a roof.  The basal fluffy part of the feather provides insulation, while the visible portion of the contour feather can be colorful, plain, or a plumage pattern that helps the birds blend in the environment.  The contour feathers on the wing are called coverts.

Afterfeathers

The body feathers of a bird typically include a secondary feather called the afterfeather. The afterfeather emerges from the rachis’ underside, where the first basal barbs of the vane branch off. The afterfeather has the appearance of a typical feather. The primary function of the afterfeather is to enhance insulation.

The Ptarmigans exhibit a variation in the length of the afterfeather in their summer and winter plumages. They have longer afterfeathers in the winter plumage and shorter and less complex ones in the summer plumage when insulation is not vital to the bird.

Down Feathers

Unlike firm-vaned feathers, down feathers are soft and fluffy and provide excellent natural and lightweight thermal insulation. A down feather typically lacks a rachis, is somewhat flexible, and has barbs and barbules that extend directly and loosely from the basal calamus. Down feathers are also known as plumulaceous feathers. Downy barbules in down feathers tangle loosely with each other, trapping air in an insulating layer next to the skin.

Birds such as ducks, whose natal down covers their entire bodies, are said to be ptilopaedic. On the other hand, psilopaedic chicks, such as a hatchling sparrow, have only a few scattered feathers on specific tracts of the body. The down feathers of adult birds are called definitive down feathers and vary from thick continuous distribution underneath the contour feather coat to a restricted distribution on the feather tracts.

Semiplumes

Are intermediate in structure between down and contour feathers. They have a long rachis with loose plumulaceous vanes. Some are close to down feathers in structure, whereas others more closely resemble contour feathers. Semiplumes always have a much longer rachis than any long barb. Semiplumes are found at the edges of contour feather tracts but are usually hidden from view. Semiplumes enhance thermal isolation and fill out the aerodynamic contours of the body plumage.

Filoplumes

Filoplumes are hair-like feathers distributed inconspicuously throughout the bird’s body. They are associated with the contour and flight feathers. The filoplumes monitor the movement and position of vaned feathers. They are most numerous near mechanically active or moveable feathers such as the flight feathers. The calamus or quill of each flight feather may have eight to 12 filoplumes associated with it.

Filoplumes consist of a fine shaft that thickens distally, ending in a terminal tuft of one to six short barbs with barbules. The slightest disturbance of a filoplume’s enlarged tip is magnified and transmitted by the long thin rachis to the sensory corpuscle at its base. The disturbance is then sent to the muscles at the bottom of the vaned feather, causing them to adjust the feather position. Filoplumes aid fine adjustment of the remiges during flight. As might be expected, filoplumes are absent in ostriches and other flightless birds.

Bristles

Bristles are another specialized type of feather with both sensory and protective functions. Bristles are simplified feathers that consist only of a stiff, tapered rachis with a few basal barbs. Bristles have sensory functions, as they have sensory corpuscles at their base.

Semi bristles are similar but have more side branches. Except for a few differences, bristles are found only on the bird’s head. The facial feathers of raptors have simplified bristles and semi bristles, which are easier to keep clean than fully vaned feathers.

Bristles are present on the head of carrion-eating birds, which have bare heads with scattered bristles.
The eyelashes of such birds as hornbills, rheas, and cuckoos consist of protective bristles, as do the nostril covering of woodpeckers, jays, and crows. Most aerial insect-eating birds have bristles and semi bristles around their mouths. The semi bristles around the mouth of nightjars are well-developed, acting not only as an insect net but possibly also as sensors of tactile information, like a cat’s whiskers.


Bird preening its body and wing feathers

Feather Care


Daily care of the feathers is essential. Birds may preen their feathers several times an hour while resting. They systematically rearrange their plumage with their bills, repositioning out of place feathers. They also draw the long flight feather individually and firmly through the bill to restore the vanes’ integrity. Birds groom and get rid of parasites on the head and neck feather by vigorously scratching. Herons nightjars and barn owl have miniature combs on their middle toe that are used in grooming.

Most birds scratch their head directly, reaching up under the wing afoot though some scratch indirectly over the wing. The advantage of one method over the other is not apparent. Related birds tend to scratch alike. For example, sandpipers scratch directly, whereas plover and the related oystercatchers and stilts all scratch indirectly.

Feathers are made of inert keratin and do not have an internal system of nourishment and maintenance. If it were not for regular maintenance, they would become brittle with exposure to the sun, water, and wind. Bird apply waxy secretions of the uropygial or preen gland located on the rump at the base of the tail. This gland is found in most birds and appears to have evolved as an essential accessory to maintain feathers’ function.

The preen gland secretes a rich oil of waxes, fatty acids, and water, which, when applied externally with the bill, cleans feathers and preserves their moistness and flexibility. The preen gland’s secretions do not alter the insulative properties of the semiplumes and down feathers. Regular application of the secretion to the plumage sustain its waterproof and insulating function. Waterbirds typically have large preen glands but whether the secretion of this organ is essential for keeping their feathers dry and maintaining buoyancy is uncertain.

The waxy secretion of the preen gland also helps regulate the bacterial and fungal flora of the feathers. Specific preen gland lipids protect feathers against fungi and bacteria that digest keratin, promote non-pathogenic fungi’ growth, and discourage feather lice. Such chemical hygiene is one of the most critical functions of the preen gland secretion.

Feather Growth

Although a bird preens and maintains its plumage’s position and function, it can’t change an individual feather’s structure. Once full-grown, feathers cannot change color or form except through fading or abrasion. No nerves muscles or blood vessel lies beneath the outer surface of the exposed feather. The only mechanism to repair damaged feathers is the replacement of the whole feather. Except in case of an accidental loss, feather replacement takes place with age and with seasons.

New feathers grow from specialized pockets of epidermal and dermal cells called follicles. The new feather grows rapidly, and toward the end of its growth, the basal cell forms a simple cylindrical calamus that anchors the follicle’s mature feather. The emerging feather then pushes its predecessor out of the follicle.

The transformation of soft epidermal tissue into a hard, durable structure is the last phase of feather growth. A major shift in the cell function causes the next feather to fill with keratin. As the process of keratinization ceases in the oldest outermost feather cells, the sheath encasing the young feather cracks, allowing the keratinized feather tips to emerge and the barb to unfold. The pulp, the core of living cells and blood vessels, is then reabsorbed by the follicle, rendering the completed new feather inert. The follicle grips the feather at the calamus by a combination of muscular tightening and friction. Substantial force is required to pull a feather from their grips.

Final Remarks

Feathers are unique to birds and are essential for insulation, aerodynamic power necessary for flight, and communication among members of the same species. Bird feathers are used as a camouflage cover to avoid detection by predators. Birds have developed feathers for sound production, tactile sensation, organ protection, and courtship display.


References:

  • Bonser, R.; Saker, L.; Jeronimidis, G. (2004). “Toughness anisotropy in feather keratin.” Journal of Materials Science. 39 (8): 2895–2896.
  • All About Birds: https://www.allaboutbirds.org/
  • Bonser, R.H.C.; Dawson, C. (1999). “The structural, mechanical properties of down feathers and biomimicking natural insulation materials.” Journal of Materials Science Letters. 18 (21): 1769–1770.
  • Delhey, K.; Peters, A.; Biedermann, P. H. W.; Kempenaers, B. (2008). “Optical properties of the uropygial gland secretion: no evidence for UV cosmetics in birds”. Naturwissenschaften. 95 (10): 939–46.
  • Delhey, K; Peters, A.; Kempenaers, B. (2007). “Cosmetic coloration in birds: occurrence, function and evolution” (PDF). Am. Nat. 169: S145–158.
  • Dimond, C. C., R. J. Cabin and J. S. Brooks (2011). “Feathers, Dinosaurs, and Behavioral Cues: Defining the Visual Display Hypothesis for the Adaptive Function of Feathers in Non-Avian Theropods.” BIOS. 82 (3): 58–63.
  • Foth, C. (2011). “On the identification of feather structures in stem-line representatives of birds: evidence from fossils and actuopalaeontology.
  • Grande, J. M.; Negro, J. J.; Torres, M. J. (2004). “The evolution of bird plumage colouration: A role for feather-degrading bacteria?” (PDF). Ardeola. 51 (2): 375–383.
  • Prum, Richard O. & AH Brush (2002). “The evolutionary origin and diversification of feathers” (PDF). The Quarterly Review of Biology. 77 (3): 261–295.

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