THE FEATHERS
Feathers are highly specialized epidermal structures that have evolved from the scales of reptiles. Although
Figure 37-6 Wing feathers of a pigeon.
There are 10 primaries or hand feathers and 10 secondary or arm feathers.light in relation to their size, they are of sturdy construction. Six types are recognized (contour feathers, semiplumes, filoplumes, down feathers, afterfeather, and bristles), but only the contour and down feathers are described here. The former are the externally visible feathers that modify the body contours, the wings, and the tail and are the feathers of flight. The contour feathers (tectrices) conceal the down feathers, which create an effective dead air space that insulates the body. The wing feathers (remiges) are made of approximately 10 primaries or hand feathers (Figure 37-6) and 10 to 20 secondaries or arm feathers. The tail feathers (rectrices) are attached to the pygostyle and are used for steering and braking during flight. There are usually 6 pairs, but numbers can vary from 4 to 10 pairs depending on the species. Feathers are concentrated in tracts (pterylae), leaving bare areas (apteria) that are preferred surgical sites. Feathers hide emaciation.
The exposed portion of a typical contour feather consists of a main shaft extended on each side by the vane (Figure 37-7). The vanes in wing feathers are asymmetrical; the external side of the vane is narrower than the internal for aerodynamic flight. The vane consists of numerous closely ranked branches (barbs; Figure 37-7/2) that leave the shaft at angles of about 45°. Adjacent barbs are connected by large numbers of minute barbules to form the level surfaces of the vane. This connection is effected by microscopic hooks on the distal ranks of barbules that loosely engage the proximal barbules crossing under them (Figure 37-7/3').
Neighboring barbs are easily disconnected but reattach if brought together, as in preening or grooming the feathers.The main shaft on the undersurface of the feather presents a longitudinal groove that ends in a depression (distal umbilicus; Figure 37-7, B/8) opposite the fluffy proximal part of the vane. A small downy afterfeather (hyopenna; Figure 37-7/9) may emerge from the umbilicus and contribute to the fluffiness.
Figure 37-7 A, Contour feather. B, Down feathers (with enlargements). 1, Main shaft; 2, barb with barbules; 3, distal barbules with microscopic hooks; 3', proximal barbules; 4, vein formed by the barbs; 5, quill; 5', quill in feather follicle; 6, dermal papilla; 7, feather muscle; 8, distal umbilicus; 8', proximal umbilicus; 9, afterfeather.
The embedded part (quill, calamus) of the feather occupies the feather follicle, an oblique tubular invagination of the skin (Figure 37-7/5'). The small dermal papilla at the bottom of the follicle extends into the opening (proximal umbilicus) at the proximal end of the quill (Figure 37-8). The quill itself is hollow and contains air and cellular debris (pulp caps) derived from the papilla. Feather muscles (Figure 37-7/7), similar to the mammalian arrector pili muscles, attach to the sides of the follicles; they often form extensive networks that elevate or lower whole groups of feathers.
The barbs of the down feathers (Figure 37-8) do not interlock to form a closed vane. Their haphazard arrangement gives these feather their fluffy appearance. In pigeons and many psittacines, like cockatoos and African greys, specialized down feathers produce a fine powder keratin dust from the barbs. This talc-like
Figure 37-8 Dermal papilla (1).
powder is then coated over the plumage during preening; its absence is often the first sign of the circovirus infection (psittacine beak and feather disease).
In pigeons the production of powder down has been associated with human allergic alveolitis or pigeon fancier’s lung.Feather color plays a major role in camouflage, courtship, and protection from heat and light. Color, produced by pigments and by the intersection of light with feather structure, may complement other features of the feathers or feather tracts in indicating sex. Other species are monomorphic, and in them sex determination is dependent on endoscopy or molecular techniques (DNA analysis).
The black pigment melanin, which also produces greys and browns, is that most commonly found in birds; it is synthesized from tyrosine. The red, orange, and yellow caretenoid pigments that produce such colors as cardinal red and flamingo pink are obtained from the diet. Porphyrins, nitrogenous pigments also synthesized by birds, provide green, red, and some browns. They occur in gallinaceous birds, pigeons, and owls and may fluoresce when exposed to UV light.
Blue pigments are not found in birds, but the color may appear when white light is scattered by feathers that absorb the red (short wavelength) end of the spectrum while the blue end is reflected—the so-called Tyndall effect responsible for the blue of the sky. More green is produced by the combination of this effect with a yellow caretenoid pigment than is produced by porphyrins. The iridescence seen in starlings and peacocks is produced by a combination of melanin with the structural breakdown of light striking the feather barbules. The color varies with the angle from which it is viewed.
At set times birds replace their feathers (molt or ecdysis) to discard worn ones or to change their plumage for display or camouflage. This occurs usually once a year after the breeding season (postnuptial or winter plumage) and is induced by the thyroid hormone. Other factors influencing molting are nutrition, time of year, temperature, and light. It also depends on habitat and whether the species is migratory.
Young birds molt their juvenile feathers before they become adults and often go through a series of subadult plumages. During molt, which is a slow and gradual process, birds should not be stressed; they require rest and a diet rich in protein (especially the amino acids lysine, cystine, and arginine) and minerals (calcium and iron) to support the higher metabolic demands (increases of 15% to 25%) made by the rapid epidermal proliferation and loss of insulation. Birds in poor condition often produce misshapen feathers. In most species, replacement of the large contour (flight) feathers is sequential (inside primaries first) and symmetrical so that flight always remains possible. Ducks and geese, however, lose these feathers at once, leaving them temporarily flightless. The old feather is pushed out by epidermal growth at the base of the follicle, and as it vacates the follicle, its replacement begins to grow. Before the barbs are released, they are encased in a sheath called a bloodfeather or pinfeather. The loss of a feather by plucking initiates a similar sequence of events. Clipping feathers is therefore unlikely to permanently disable birds for flight.