Avian Anatomy: Chapter 4 on Feathers

Chapter 4: Feathers

• Feathers are the most numerous, elaborate, and diverse derivatives of the avian integument.
• Feathers are an extraordinary evolutionary innovation and the most distinctive feature of avian anatomy.
• Feathers collectively referred to as plumage are the most complex structures to grow out of the skin of any vertebrate, and feathers provide a rich diversity of functions in the lives of birds.
• Insulation for controlling body temperature, aerodynamic power for flight, and colors for communication and camouflage is provided by feathers.
• Modified feathers perform secondary roles, including those in swimming, sound production, hearing, protection, cleanliness, water repellency, water transport, tactile sensation, and support.
• Male Great Argus, an Asian pheasant, uses its spectacular wing feathers to blow fallen leaves off of its display court.
• Feather suppression requires regular preening, including application of oily secretions of the preen gland.
• Seasonal molts replace worn feathers with new ones and sometimes replace cryptically colored feathers with colorful ones.

Feather Structure

• Feathers consist mainly of beta-keratin, a fibrous protein polymer that forms microscopic filaments with strong mechanical properties.
• Beta-keratins are unique to birds and other reptiles, have similar mechanical properties to alpha-keratins found in all vertebrates, including humans and birds, but are an entirely unrelated family of proteins with a very different molecular structure.
• Beta-keratins make up most of the hard structures of reptilian skin, the leg scales, claws, and beaks of birds.
• Feather keratins are a special class of beta-keratins characterized by a small deletion in their molecular sequence.
• Contour feathers constitute the outline, or contour, of the body.
• Typical contour feather features have a long central shaft and a broad, flat, planar vane on either side of this shaft.
• The tubular, hollow base of the shaft-the calamus, or quill, anchors the feather into the follicle in the surface of the skin.
• The rest of the shaft-the rachis-supports the feather vanes.
• Lateral branches off the rachis, called barbs, are the primary branches of the vane.
• Each barb consists of a tapered central axis, called the ramus (pl. rami), with rows of smaller branches, called barbules, projecting from both sides.
• The multicellular barb rami are composed of an outer layer of flattened cortical cells, that are solid keratin, around a spongy core of larger, box-shaped medullary cells which are empty and air-filled.
• Spongy medullary cells make barbs structurally strong and resistant to bending.
• Each barbule consists of a series of single cells fused end to end; cells may be simple or may bear projections called barbicels, which may be elaborate and hooklike.
• Barbs and barbules interlock to form the coherent but flexible surface of the pennaceous feather vane.
• Distal barbules, extending toward the tip of the feather vane, feature tiny hooklets.
• Proximal barbules, extending toward the base of the feather, have prominent grooves.
• Many contour feathers have a fluffy, downy, or plumulaceous portion of the vane that is usually hidden, deep within the plumage.
• Downy barbules on the barbs at the base of the body feather are long, thin, and flexible and have small nodal prongs at the junctions of neighboring barbule cells.
• Downy bases of contour feathers provide insulation.
• The shape, distribution, and pigmentation of nodal prongs of down feathers provide critical evidence to feather forensics.
• The contour feathers of some birds include a secondary structure — an afterfeather — which is a mirror-image rachis and vane, attached to the same calamus
• The barb and barbule structure of afterfeathers is typically plumulaceous.
• When the afterfeather is reduced to a simple rachis, it is called an aftershaft.
• The afterfeather's primary function is to enhance insulation.
• The afterfeathers of the winter plumage of Ptarmigans, grouse of high, cold alpine habitats, are three-fourths as long as the main feathers and provide essential insulation.
• Tiny, flat contour feathers that cover a penguin’s body create a smooth, almost scaly surface that reduces friction during swimming.
• Vane shapes range from long and pointed display feathers (those on a rooster's neck) to short and round, like the head and facial feathers of small birds.
• Large barbs with extra-long, curved barbicels produce water-repellent feathers in petrels, rails, and ducks.
• The loss of these barbicels on contour feathers of cormorants and anhingas is an adaptation for diving.
• Coiled barbules on the belly feathers of sandgrouse help them transport water to their nestlings.
• The flight feathers include long, stiff, pennaceous, wing feathers (remiges) and tail feathers(rectrices).
• Because of the aerodynamic forces necessary for flight, flight feathers have asymmetrical vanes in which the leading-edge vane of the feather is narrower than the trailing vane.
• Flight feathers have little importance in insulation, and flight feathers lack an afterfeather.
• Outer (distal) remiges that attach to the bones of the hand and the second digit are called the primaries.
• Inner (proximal) flight feathers of the wing that attach to the trailing bone of the forearm, or ulna, are called the secondaries. -Most birds have 10 primaries.
• Storks, flamingos, grebes, and rheas have 11 primaries.
• Ostriches have 16 primaries.
• Some songbirds have nine primaries.
• Flightless Kiwis have only three or four primaries.
• The secondaries vary in number from six in hummingbirds to 19 in some owls and 40 in albatrosses.
• Primaries are strongly asymmetrical in shape with the leading-edge vane narrower than the trailing vane.
• Distal barbs on the leading edge vanes of the owl’s primaries have very long, filamentous tips, called pennulae (sing. pennulum) that create a fuzzy layer on the obverse surface of the vane that reduces air turbulence, especially at low speeds.
• Usual 12 rectrices function primarily in control, steering, and braking during flight.
• Tail feathers can also be modified for sound production in snipes, or for bracing support in creepers, woodpeckers, woodcreepers, swifts, and penguins.
• Down (plumulaceous) feathers are soft and fluffy.
• Down feathers vary from thick, continuous distribution in some chicks to restricted distribution among the other feathers in adult birds.
• Down feathers provide excellent lightweight thermal insulation and water repellency.
• Down feathers of chicks, called natal down, typically lacks a rachis, but there are exceptions with waterfowl.
• Natal down feathers grow from the same follicles that will later grow pennaceous contour feathers.
• Adult downs typically grow from specific follicles.
• Plumalaceous portions of contour feathers and barbule cells of down feathers have tiny nodal prongs.
• Downy barbules entangle loosely, trapping air in an insulating layer next to the skin.
• Semiplumes are intermediate in structure between down and contour feathers.
• Semiplumes enhance insulation, fill out aerodynamic contours, and serve as courtship ornaments.
• Filoplumes are a very distinct class of hairlike feathers that function in sensing the movement and position of adjacent, vaned feathers.
• Filoplumes consists of a fine rachis with a terminal tuft of one to six short barbs with barbules at the tip.
• Filoplumes provide the bird with sensory information. -Filoplumes are absent in flightless penguins and ostriches.
• Bristles are specialized feathers with sensory and protective functions.
• Bristles are simplified feathers that consist only of a stiff, tapered rachis with a few basal barbs.
• Like filoplumes, many bristles have sensory corpuscles around their follicles.
• Except for those on the knees of the Bristle-thighed Curlew and on the toes of some owls, bristles are usually found on the heads of birds.
• The facial feathers of raptors are simplified to bristles and semibristles, which are easier to keep clean than are fully vaned feathers.
• Powderdown feathers produce dustlike beta-keratin particles about one in diameter that resemble talcum powder.
• Birds disperse the oily powder over the entire plumage as they preen their feathers.

Feather Development

• Feathers are dead structures when mature, and after they are fully grown, cannot change color or form except through fading or abrasion.
• The first feathers develop on the embryo within the egg.
• Feathers are replaced through a regular, periodic molt throughout the life of the bird.
• Individual feathers may be replaced if they are accidentally lost or damaged.
• Feathers grow from specialized organs in the skin called follicles.
• The outer layer of the skin, or epidermis, is composed of cells that will keratinize and die when they mature.
• The inner layer of the skin, or dermis, provides nutrients and developmental signals to the epidermis.
• The follicle consists of a tubular in-pocketing, or invagination, of the epidermis.
• All feathers are the tubular outgrowths of the inner, ascending, epidermal layer of the follicle.
• At the base of a follicle, where the epidermis turns, is the follicle collar, a persistent ring of feather stem cells that will divide to produce the cells of the feather.
• Feather placodes consist of tiny thickenings of the epidermis that determine the site where the follicle will develop.
• At the feather germ, the epidermal cells use cell-cell signaling proteins to coordinate their differentiation into the various feather parts.
• Outermost epidermal layer of cells becomes sheath which falls off when feather growth is complete.
• Intermediate cells become divided, or compartmentalized, into barb ridges which form major branches of feather vane.
• The fusion of barb ridges on the dorsal side of the tube forms the rachis ridge, which becomes the rachis of the mature feather.
• Dermal pulp is the core of living cells and blood vessels at the center of the growing feather, and it is periodically resorbed by the dermis of the follicle.
• As the feather grows, the dermis produces pulp caps, keratinized lids that keep the dermis from leaking out the tip of the feather germ.
• The inferior umbilicus is a small hole at the bottom of the calamus which is a vestige of the space filled by dermal pulp in the growing feather and provides evidence of the essential tubularity of the feather.
• A tight grip of follicle muscles controlled by the autonomic nervous system may relax when a bird becomes mortally frightened
• Fright molt where the resulting loss of feathers (shreckmauser in German) is hypothesized to be an extreme antipredator adaptation

Evolution of Feathers

• Feather development informs a developmental theory of feather evolution, and the rachis is formed by the fusion of barb ridges implies that barbs evolved before the rachis.
• Feathers evolved through five distinct stages, and each required a new mechanism of growth or a developmental novelty as feathers evolved their diversity and definitive form.
• The diversity of the feathers of nonavian theropod dinosaurs also support the predicted early stages of the developmental theory of feather evolution.
• Primitive and completely modern feathers from numerous lineages of theropod dinosaurs demonstrates feathers first evolved in bipedal, terrestrial, meat-eating theropod dinosaurs before the origin of birds and before the origin of avian flight.

Feather Colors

• Feather colors come in many shades, hues, and tints because of organic pigments deposited in the feather cells and nanometer-scale structures of feather cells.
• Pigments contribute to visible colors by absorbing the wavelengths of light complementary to the color perceived.
• Structural colors result from the physical, optical interactions between incident light and feather nanostructures.
• The four major classes of feather pigments are melanins, carotenoids, psittacofulvins, and porphyrins.
• Melanins produce earth tones.
• Carotenoids produce bright yellows, oranges, reds, and purples.
• Psittacofulvins produce the yellow, orange, and red feather colors in parrots only.
• Porphyrins are responsible for unique, bright, olive green and magenta plumage colors in turacos and a few other birds.
• Melanin pigment is synthesized from the amino acid tyrosine by mobile pigment cells called melanocytes, which creep about in the dermal layer of the skin.
• Like gravel in concrete, melanosomes make feather keratin more resistant to mechanical stress and wear.
• 39 different carotenoid molecules have been described from bird plumages.
• Psittacofulvins are simple hydrocarbon chains of 14 to 20 carbons with seven to 10 double-bonded carbons that are very similar in structure to the central chain of a carotenoid.
• Magenta red pigment in the wing feathers of many turacos is produced by a copper-contained porphyrin called turacin, or uroporphyrin III.
• Spongy medullary structural colors that come in two distinct spatial varieties are channels and spheres. Bright green plumage colors are produced by a combination of spongy medullary structural color with a yellow carotenoid or (in parrots) psittacofulvin pigment.
• The feather colors of birds, especially blues and violets, are rich in UV reflectance, between 320 and 400 nm in wavelength.
• Mutation in the melanocortin-1 receptor gene influences melanin pigmentation.

The Plumage

• A Tundra Swan has roughly 25,000 feathers.
• Songbirds typically have from 2,000 to 4,000 feathers.
• A Bald Eagle weighs about 700 grams, more than twice as much as its skeleton (272 grams) and between 17 and 20 percent of its total adult body mass.
• Feather attachments are densely grouped in particular areas called feather tracts, or pterylae, which are separated by regions of skin with few or no contour feathers, called apteria.
• Alar, Capital, Ventral, Humeral, Femoral, Crural and Caudal areas make up a birds major feather tracts
• Most birds have a ventral apterium, which facilitates incubation of their eggs.
• Penguins lack apteria entirely, perhaps as an adaptation to cold climates and water.
• Pairs of tiny networks of antagonistic muscles interconnect the follicles of neighboring feathers.

Feather Care

• Birds preen their feathers with their beaks and toes to maintain their pennaceous feather vanes and to fight parasitic mites, lice, and bacteria
• Vital preening function of the avian beak may have constrained the evolution of avian beak shapes.
• Miniature combs on the inner margins of the claws of middle toes, used for grooming, have evolved in herons, nightjars, and barn owls.
• Many bird species also preen their mates or other social partners — a behavior called allopreening.
• Birds apply to their feathers the waxy secretions of the uropygial gland, or preen gland, located on the rump at the base of the tail.
• A rich oil of waxes, fatty acids, fat, and water are what is secreted by the preen gland, which, when applied externally with the bill, cleans feathers and preserves feather moistness and flexibility.
• Regular applications of the secretion to the plumage sustain its functions as an insulating and waterproofing layer
• Feather ectoparasites include chewing lice, louse flies, and feather mites.
• Feather-chewing parasites damage the structural integrity of feathers, which can reduce both the winter survival and the attractiveness of male pigeons.
• Sister species of feather lice that live and feed on the Yellow-crested Cockatoo and Yellow-tailed Black Cockatoo have rapidly diverged in color to be either brilliant white or dark black to match the plumage colors of their host
• Homobatrachotoxin, which was first discovered in poison dart frogs from the Neotropics, is contained in pitohui shrike-thrushes.

Plumage Color Patterns

• Plumage coloration varies in pattern from cryptic (concealing) to bold.
• The value of contrast increases with the intensity of illumination from above, so open-country birds, such as plovers, often have strongly contrasting colors on their upper and lower surfaces.
• White underparts work particularly well as a neutral (achromatic) reflector that takes on the hue of the nearest surface.

Molts and Plumages

• A bird's feather follicles and its first feathers develop while the embryo is still in the egg.
• Birds proceed through a series of specific molts from natal down feathers of the embryo and the first juvenal plumage.
• Basic plumage is produced by a single annual prebasic molt.
• Some birds that have a different appearance in the breeding and nonbreeding seasons have evolved an additional prealternate molt before the breeding season, which gives rise to the alternate plumage.
• Typically, the alternate and basic plumages are displayed during the breeding and nonbreeding seasons, respectively.
• Body molt proceeds in a characteristic spatial and temporal wave over the body.
• The molt of flight feathers must proceed in regular sequence, feather by feather, in order to avoid big gaps in the wings that would compromise flight capacity.
• Waterfowl (Anatidae) and Magpie Geese (Anseranatidae) are unique in exhibiting simultaneous molt of all of their flight feathers after breeding, rendering them flightless.