Feathers

Adult or definitive down feathers of adult birds are extremely plumulaceous feathers that provide a layer of insulation underneath the contour feathers. Down feathers either lack a central rachis or sometimes have a very short rachis, shorter than the longest barbs. The barbs sometimes attach directly to the basal calamus of the feather. Down is not evenly distributed, and some groups (sea ducks, for example) have much heavier down coats than other groups (such as songbirds).

Bristles are hairlike contour feathers without vanes. They consist only of a whiskery central rachis almost bare of barbs or barbules. Not all birds have bristles (for example, the rock dove has none). Bristles are found mostly around the eye (for protection), lores, nostrils, and the rictus of the mouth (rictal bristles). Insectivorous birds are thought to use their prominent rictal bristles as sensory organs, much the way mammals use whiskers.

Contour feathers are the basic vaned feathers of the body and wings and include the large flight feathers of the wing and tail. Smaller contour feathers cover the body and have a symmetrical vane dividing them between a firm, pennaceous (having a central shaft or rachis with vanes branching off to either side) distal vane area and a soft, plumulaceous (downy) inner vane area. In some birds, the contour feathers of the body tend to have more prominenet afterfeathers than do the flight feathers.

Filoplumes are long, hairlike feathers that monitor the position of the pennaceous feathers, as those of the wings and tail. Sensory corpuscles at the base of each filoplume detect fine movements of the filoplume shaft. Filoplumes are often numerous at the bases of wing remiges to monitor the position an movement of the remiges during flight. In many passerines, they also protrude through the outer contour feathers of the crown and nape, perhaps the warning the bird when wind disrupts the smoother outer surface of the plumage.

Natal down or neossoptiles cover hatchlings. Simpler than adult down, the feathers rarely have a central rachis (except in ducks). The barbs are also simpler, having fewer barbules. Often natal down is immediately pushed out of the feather follicle by the emerging juvenile plumage and appears as a tuft at the tip of a new feather.

Powder down consists of special feathers with barbs that disintegrate into a fine powder. They are thought to aid the bird in grooming and waterproofing its feathers. They are the only feathers that grow continuously and are never molted. Many species have widely scattered powder feathers within patches of normal down feathers, but herons and bitterns have dense, prominent patches of powder feathers on the breast and belly.

Rectrices (singular, rectrix) are the large, vaned flight feathers of the tail. Similar in structure to the remiges of the wing, rectrices also have asymmetrical vanes. In some groups, as woodpeckers, the rectrices have adapted and strengthened to act as props, helping birds remain vertical as they forage on tree trunks. Swifts use similar stiff rectrices as an aid in perching on vertical surfaces.

Remiges are the flight feathers of the wing and include the primaries, secondaries, and tertiaries. Remiges (singular, remex) are pennaceous contour feathers with prominent, often asymmetrical, vanes. In ducks, gallinaceous birds (usually ground-feeding domestic or game birds), and owls, the ventral vane surface is partially modified into a shiny, firm structure formed by specialized tegmen feather barbs, which are believed to strengthen the vane and resist the flow of air upward through the vane surface.

Semiplumes are between the more pennaceous contour feathers and the strictly plumulaceous down feathers, which lack a central rachis. Semiplumes always have a distinct rachis that is longer than any of the barbs. Seldom exposed, semiplumes lie under the surface of contour feathers, insulating the body and forming smooth, aerodynamic body contours.

A typical contour feather is composed of:

• a long central shaft
• a broad flexible vane on either side of the shaft

The calamus or quill is the bare part of the central shaft. It is hollow, in the mature feather, with a small opening at the proximal tip called the inferior umbilicus, which is sealed with a plate of keratin.

In the lumen of the calamus are several internal pulp caps which are remnants of the pulp tissue that nourished the growing feather early in its development.

The superior umbilicus is another small opening of the calamus area. It is a remnant of the distal end of the epithelial tube that forms early in feather development.

The rachis is the central feather shaft. In the major flight feathers, the rachis is distinctly grooved on the ventral surface and is roughly squared when viewed in a cross-section. This grooving gives the outer feather shaft added strength in bending.

On either side of the rachis are two asymmetrical vanes. The forward (anterior) vane of the primary remiges is always narrower than the trailing (posterior) vane. This gives the feather an airfoil cross-section and each primary or secondary feather can act as an individual in flight. The smaller contour feathers that cover the body do not need to act as airfoils and are, therefore, symmetrical. The proximal end of the vane is plumaceous (soft and downy) while the distal vane is pennaceous (firm and bladelike). The feather vane is made up of hundreds of tiny parallel barbs that branch off from either side of the central rachis.

Bird feather colours are produced by a variety of pigments and structural adaptations of the feathers.

• Melanins are the most common and produce black, gray, and brown. They are synthesized through oxidation of the amino acid tyrosine. Melanin granules produce colour in direct proportion to their presence in the feather; the more melanin, the darker the colour. Melanins can be found in all types of feathers, especially the major flight feathers. The pigment eumelanin produces the black, gray, and dark brown while phaeomelanin produces light brown, brick red, and dull yellow or tan.
• Carotenoids produce intense reds and yellows. They are derived exclusively from the bird’s diet, mainly the yellow carotene pigmets in grains, seeds, and other vegetable matter. Carotenoids are rarely seen in flight feathers but mainly in back and breast plummage and only in the contour and simiplume feather types. The pigment lutein (a xanthophyll), zeaxanthin, and beta carotene form the bright yellows. Astaxanthin, rhodoxanthin, and canthaxanthin form the bright reds.
• Porphyrins produce a range of red, brown, green, and the brown pigment of many owls. These feather pigments are related to hemoglobin and other bile pigments formed by the breakdown of hemoglobin by the liver. The most common porphyrins produce brown pigments but can also produce the bright reds and greens seen in turacos and a few other species. The pigment turacoverdin produces green while turacin (uroporphyrin) produces red, and coproporphyrin III produces brown and reddish-brown.
• Blue and green birds usually have no blue or green pigment in their plummage but rather are able to create complex patterns of reflection and refraction in the cell walls at the surface of the barbs and barbules of each feather. For example, the blue jay (Cyanocitta cristata) is not really blue but reflects only the blue light wavelengths.
• Iridescent colours result from an interaction of the microstructure of the feather and melanin granules imbedded in the barbules of each feather. In some species, iridescence results from the many laminations of keratin layers, with each reflecting different wavelengths of light. Hummingbirds have an elaborate layering of reflective melanin granules, and colours are determined by the angle of these layers relative to the viewer’s eye.
• Structural colours result from the modification or separation of the components of white light by the structure of the feather. In white feathers, the whole feather structure simply reflects back the whole colour spectrum. Blue is usually structural and rarely results from pigmentation. Green is usually structural but can sometimes be from a combination of yellow carotenoids and black melanins. Combinations of structural and pigment colours are common, particularly in yellow-green, green, and blue-green feathers. Iridescence is caused by the complex layering of cell walls or melanin granules in the barbules of feathers. These colours selectively absorb or reflect varying wavelengths of light and exact colours will depend on the viewer’s eyesight. Iridescence is primarily structural although melanin granules are almost always abundant in iridescent feathers.

In most birds, the contour feathers of the wings and body are not uniformly distributed over the skin surface. This pattern of distribution is called pterylosis. Follicles of the contour feathers are concentrated in dense tracts called pterylae (sing. pteryla) and separated by bare areas called apteria (sing. apterium).

In such birds as flightless penguins (Family Spheniscidae) and ratites like the ostrich (Family Struthionidae) and the rheas (Family Rheidae), contour feathers are evenly distributed and not organized into tracts. toucans (Family Ramphastidae) also lack apteria but this is thought to be adaptation rather than an ancient characteristic.

Pterylosis, although characteristic within certain taxonomic groups, can vary greatly depending on the family, genus, or even species of bird. Some, that have areas barren of contour feathers, will almost always have some down or semiplume quills. In highly aquatic birds, the apteria are considerably reduced and may be densely feathered with thick down to insulate the body from cold water.

Capital tract extends over the entire dorsal surface of the head (pileum) from the base of the maxilla, over the forehead and crown areas (including crest feathers), and down the back of the skull to the nape of the neck where the skull meets the spinal column. The lateral boundary passes below the external ear opening and therefore includes auricular feathers, rimal feathers around the eyelids, rictal bristles, and all other specialized feather regions of the head.

Spinal tract is actually a complex and highly variable series of tracts and apteria lumped together into one group. It runs down the dorsal midline of the body from the base of the skull to the pygostyle at the posterior end of the vertebral column. In most birds, it is simply the tract that runs dorsally along the midline of the back. In such birds as the Rock Dove, the length is split, with a narrow apterium along the dorsal midline. The spinal tract is bordered along its length by the cervical apteria of the lateral neck, the scapular apteria at the shoulders, and the broad lateral apteria along the lateral walls of the abdomen.

Ventral tract covers the ventral neck, breast, and abdominal regions. It courses posteriorly along the ventral midline of the body as a single tract from the notch between the mandibular rami of the lower bill to the base of the neck just anterior to the breast muscles. The tract then divides into two lateral bands (the pecterosternal tracts) that enclose a bare sternal or mid-ventral apterium along the ventral midline of the thorax and abdomen. The lateral ventral tracts reunite at a circle of feathers around the cloaca (cloacal circlet).

Caudal tract includes the major flight feathers of the tail (rectrices), which vary in number depending on the taxonomic group. In the Rock Dove, there are twelve rectrices, six on each side of the tail, as well as in most passerines. The caudal tract also includes the upper tail coverts of the dorsal caudal tract and the lower tail coverts of the ventral caudal tract.

Humeral tract is a band of contour feathers that overlie the humerus bone on the dorsal side of the wing (shoulder area of the bird).

Alar tract is made up of a series of smaller pterylae that cover both the dorsal and ventral surfaces of the outer wing. It also includes the primary, secondary, and tertial (if present) remiges (the major flight feathers) as well as the alula feathers of the second digit. The smaller alar tracts contain all of the rows of greater, middle, and lesser covert feathers that cover the wing both dorsally and ventrally, except those coverts within the humeral tracts.

Femoral tract covers the outer surface of the thigh in a diagonal strip from the knee joint upward toward the base of the tail.

Crural tract covers the rest of the leg not covered in the femoral tract.

Metatarsal tract covers the tarsometatarsal area of the lower leg in large birds and those with heavily feathered legs.

This page was updated in June 2006.