PLUMAGE. Feathers are pre-eminently the distinguishing characteristic of birds: all birds have them, while no other animal pos sesses them. As elsewhere described (see ORNITHOLOGY) feathers fill two important func tions, the retention of animal heat in the body and the support of the bird in flight; and since they cover practically the whole bird, all the striking features of color and ornament so highly developed in this group, pertain to the feathers.
The typical or pennaceous feather consists of a hollow nearly transparent, cylindrical portion (calamus), which merges above into an apaque, usually squared shaft (rhachis). The calamus, or quill, is composed of a series of oblong cells nesting into one another and containing the from which the growing feather is built up. When the feather is complete these cells are empty. The shaft bears on either side the webs or vanes, soft flexible surfaces which upon slight tension split up into their component parts — rami or barbs. These are slender lamel la which branch off from the rhachis and in turn give rise to branches — radii or barbules. The anterior barbules of one barb overlie the posterior barbules of the next barb and the numerous cilia (barbicels) which they bear, many of which are hook-shaped at the tip, interlock with the thickened upper edge of the posterior barbules, binding the barbs together and form ing the substantial web which is so essential to flight. In the inner web of a crane's feather 15 inches in length have been counted 650 rami each bearing about 600 pairs of radii, or nearly 800,000 radii for the inner web alone and far above 1,000,000 for the entire feather. At the point where the rhachis emerges from the cala mus is a pit known as the umbilicus and here is frequently found a sort of secondary feather known as the hyporhachis or aftershaft; it is usually much smaller than the main shaft and more downy in character, but sometimes, as in the emu, is of the same dimensions and appearance.
Feathers are entirely products of the skin, being similar to the scales of reptiles rather than to the fur of animals so clothed. They do not grow continuously, but are periodically shed and replaced by other feathers. The first
feathers appear as small papillae on the skin of the embryo. Each papilla is produced by a group of cells known as the Malpighian cells, which increase and divide, part of them growing down and forming the lining of the feather follicle, the others forming the feather itself and surrounding the central mass or pulp, which is well supplied with blood-vessels and furnishes the requisite nourishment to the growing feather. The Malpighian cells which produce the feather form three layers — a thin exterior one, the sheath which covers the young feather in what is familiarly called the feather* stage and later scales off as the feather expands; a middle layer, which forms the feather itself ; and a very thin innermost layer, which covers the pulp and which persists in side of the calamus as a series of pithy caps left by the retreating pulp. This pulp is with drawn to the base of the follicle when the feather is completely grown and becomes active again when the feather is shed, or at the time of the periodical molting (q.v.).
Feathers are classified into (1) contour feathers — those which are outermost and give form to the bird; (2) down feathers,— the dense undercoating; (3) filo-plumes; and (4) powder-down feathers.
The first are usually pennaceous in struc ture, but in the various ornamental plumes, barbicels and even barbules may be wanting or reduced in number, producing a fluffy feather. The down feathers have no barbicels and usu. ally no rhachis, but consist of long fluffy barbs, all of which grow out from the tip of a very short calamus. Filo-plumes are exampled by the long ((hairs* found upon the body of the domestic fowl after the feathers have been plucked and commonly removed by singeing: show under the microscope a degenerate feather structure of barbs and barbules. In the case of some birds, however, they become very long, reaching out beyond the contour feathers and infinencing• the coloration and tex ture. Powder-downs are down feathers which are constantly disintegrating into a fine powder and are found in the plumage of various birds, notably parrots, tinamous, herons and the spe cies of Podargus.