Feathers are integumentary structures characteristic of birds. Strong, light, elastic, and waterproof, these extraordinary modifications of the epidermis are particularly fitted to the needs of animated aeroplanes. During the Jurassic Period, when, so far as is known, birds made their debut in the form of Archaeopteryx, they were already clothed with unmistakable feathers. It is probable that these unique epidermal structures are homologous with reptilian scales, although evidence for this supposition is mostly embryological, since neither comparative anatomy nor palaeontology shows any unmistakable transitional structures between scales and feathers. The apparent scales on the flipper-like wings of the seagoing antarctic penguins are not true scales but instead are miniature flattened feathers, as a close examination at once reveals.
A typical feather (Fig. 191) is an elaboration of the lifeless corneal layer of the epidermis. Its shafts hollow at the inserted end or quill, bears on its sides lateral barbs from which barbules extend. The barbules on the side of each barb farther from the quill bear hooks, like microscopic crochet hooks, which interlock with neighboring barbules (Fig. 192) forming a continuous expanse called the vane, that makes a fan-like surface resistant to air.
The germ of a developing feather appears as a papilla of dermal cells pushing the overlying epidermis up ahead of it. The base of the outgrowth, including derma and epidermal coat, gradually sinks into the skin (Fig. 193). There is thus a plug of dermal cells filling the epidermal covering, but as the epidermis, hardening and cornifying, is pushed out from the papilla region, the dermal plug withdraws little by little, leaving a hollow lifeless quill inserted in the skin (Fig. 194). The embryonic feather is, therefore, at first a tube of cornified epidermis, set in a pit of the corium. Within the tubular embryonic feather the wall down one side is considerably thickened and later becomes the shaft from which the slanting barbs of the developed feather extend on either side (Fig. 193). The wall of the tube opposite the shaft, the region of the distal tips of future barbs, is very thin. It is along this thin region that the rolled-up feather ruptures before it spreads out flat to assume its final definite form.
There are three general kinds of feathers, namely, quill, down, and pin feathers. Quill feathers may be further classified as tail, wing, and contour feathers, devoted respectively to the functions of steering, flying, and thatching. Tail and wing quills are larger and less flexible than the lighter and more delicate contour feathers that serve to fill out the unevennesses of the surface of the body, giving grace of curving outlines to the living bird. The part that contour feathers play in streamlining a bird is very apparent when one observes the scrawny body of a dead bird from which the feathers have been plucked.
There are two kinds of down feathers, namely, powder-down (Fig. 195), and nestling-down. Powder-down feathers are characteristic of certain adult birds. They are interspersed and concealed among the contour feathers, and are more abundant on the breast and abdomen of herons and birds of prey than elsewhere. The heat-retaining quality of powder-down feathers aids in the incubation of the eggs and the protection of the semi-naked nestlings. The shedding of powdery fragments, dropped from time to time, effects a sort of sanitary cleaning of the plumage of birds whose nests are particularly liable to become daubed with excreta and the remains of animal food.
Pin feathers, although superficially resembling hairs, are complemental in structure to down feathers. They have practically no barbs, consisting instead almost entirely of the shaft which is missing in down feathers that are made up of barbs and barbules. Pin feathers are scattered quite generally over the body among the contour feathers, although in certain birds, such as flycatchers and whip-poor-wills, they become localized about the mouth opening, serving as a “barbed wire” entanglement in the capture of insects on the wing. In common use, the term pin feather is also applied to the young quill feathers present, in addition to true pin feathers, after the so-called plucking of the bird.
In a quill feather interlocking of the barbules occurs only in the exposed part of the vane which is not overlapped by other feathers. During the effective downward stroke of the wing in flight, the vanes of neighboring feathers close up together, presenting to the air a common continuous impervious surface, while upon the return upstroke they separate somewhat, thus letting the air through with less resistance. An entirely different irregular arrangement is characteristic of down feathers and pin feathers, which have nothing to do with locomotion.
The plumage of a bird consists of all the feathers taken together. The first plumage of young birds is the transient nestling-down, which appears as fluffy tufts on the tips of the emerging contour feathers. In this first plumage it is the tip of the epidermal tube that frays out like a brush (Fig. 193) to form the nestling-down feather, which is fated to wear off after temporary service and be replaced by the unfolding quill feathers.
The nestling-down of the first plumage is thus replaced by the so-called juvenal plumage, which is made up of the first coat of true quill feathers. This lasts the young bird through its first winter when, in most cases, it is replaced* by the nuptial plumage that heralds the first love affair in the spring. In the following autumn, after the adventure of raising the first family has been accomplished, the nuptial plumage, now faded and shabby, is exchanged for a post-nuptial plumage. Every year thereafter that the bird lives there is a new post-nuptial plumage after the breeding season and in the case of many birds an additional nuptial plumage in the spring.
This process of ecdysis by means of which one coat of leathers is exchanged for another is called molting. When a dead feather loosens from its socket in the skin and is lost in molting, the living epidermal Malpighian cells at the bottom of the pit, backed up by nutritive resources of the blood vessels from the underlying corium, grow out into a new embryonic feather tube, which in turn unrolls to take the place of the dead feather that was lost.
Water birds, gallinaceous birds, and some birds of prey are said to be precocial, because at hatching they are quite well clothed with nestling-down, while certain other birds, such as kingfishers and woodpeckers, are described as altricial, because they are hatched almost naked, only subsequently acquiring their first coat of feathers.
Although the feather coat forms a remarkably complete covering over the body, the insertion of individual feathers in the skin is by no means equally spaced. Feathers are attached in localized patches called pterylae (Fig. 196), between which there are naked areas, apteria, covered by overlapping feathers from neighboring pterylae. No doubt apteria in such areas as the “armpits” and the inguinal region facilitate freedom of locomotion in much the same way as do loose running trunks on the legs of a sprinter. Apteria on the abdomen of a bird may also be useful during incubation, because the eggs are thereby snuggled into more direct contact with the warm body of the brooding mother.
The constancy and orderly arrangement of the various pterylae has been used by systematists in determining the relationships of different kinds of birds for purposes of classification. Ostriches, toucans, and modem penguins are apparently exceptional in that they do not in adult life show a pattern of feathers in pterylae and apteria. That this is a secondary acquisition and not a primary condition is indicated by the fact that fossil (Tertiary) penguins, and embryonic ostriches, show distinct pterylae.
Local deviations in feather arrangements, usually associated with secondary sexual characters, are of frequent occurrence, such as the crests and ruffs of various birds, and the spectacular tails of peacocks, fantail pigeons, and lyre birds. In a strain of fancy poultry known as “frizzles,” the plumage has departed from nature’s approved style by reason of the twisting of the feather shafts, but it is doubtful whether these curious frowzy birds could successfully maintain themselves out of domestication.
The shingle-like lay of the feathers is directed from the head toward the tail, thus reducing to a minimum the air resistance offered by the plumage. This orientation of the feathers also makes possible the retention under the feathers of a layer of warmed air next to the skin during rapid flight, which would be blown away if the feathers were arranged in any other fashion.
The remarkably varied colors of feathers are due to one or both of two factors, namely, chemical pigments and the physical arrangement of the elements making up the feathers. The reds, yellows, and blacks are due to pigments. The whites, blues, and iridescent colors are structural colors. The greens are usually due to a structural blue combined with a pigment yellow.
Pigments are deposited mostly in the exposed parts of feathers and only during the period of their growth. After feathers have become differentiated lifeless structures there is no way to add pigment granules to them, so that further change in color of plumage can then only occur in one of three ways, by fading of the pigment already in the feathers, by the wearing away of the particolored feather tips, or by complete ecdysis of old feathers and their replacement by new ones.
A feather appears white if no pigment is present and the polygonal cells of the barbs and barbules break up the light and reflect all wave lengths equally.
The blue color of feathers is of the type known as Tyndall blue. Many years ago Tyndall showed that the sky appears blue because minute suspended particles of dust, water, and the like scatter the short, blue waves of light while permitting the longer, red waves to pass through. Therefore, when we see light reflected from these particles or from similar suspended particles in any turbid medium (e.g., skimmed milk), we see the blue wave lengths. On the other hand, if we look at the turbid medium using only light which is transmitted through it, the material appears reddish because the short, blue waves have been scattered and the longer, red waves transmitted. The blue color of feathers is localized in the layer of cells which is just beneath the outer sheath of the barbs. Minute pores in the walls of these cells bring about the scattering of the blue wave lengths while transmitting the longer ones. As we ordinarily see feathers by reflected light, any area which brings about this scattering will appear blue.
In almost every case green feathers exhibit the same structural features as blue ones but have, in addition, a yellow pigment in the outer sheath of the barbs, the combination of the two colors producing green. A green copper pigment has, however, been extracted from the feathers of the West African “turacou,” Turacus.
Frequently contour feathers present complicated variegations of colors which combine to form patterns, involving the matching of parts of several neighboring feathers. Thus, a white wing-bar, or a spot on the breast, is in reality a baffling mosaic, made up of unequal fragmentary contributions of color from many separate overlapping feathers, which have grown independently into harmonious positions with relation to each other. No wonder that Darwin is said to have exclaimed that trying to think out how the “eye” on the dorsal feathers that constitute the peacock’s tail came about, made him actually sick!
In the process of molting, feathers in the centers of the separate pterylae are the first to fall out, and this loss with its subsequent replacement extends from these centers to the margins of the different feather islands.
Sometimes a molt is incomplete not involving each feather, but simply the wearing away of a different colored tip. This may be quite effective, however, in accomplishing a change in general appearance, as for example, in the case of the male bobolink, Dolichonyx, which changes from a distinctive coat of black and buff in patches of color in the spring to an inconspicuous streaked sparrow-like plumage in the fall.