FEATHER, a horny outgrowth of the skin of birds, homo logous with the scale of the reptile. The body-covering of all birds is composed of feathers, and by this character alone birds may be distinguished from all other animals.
The most perfect form of feather is made up of a long, taper ing rod, fringed on either side, for the greater part of its length, by a secondary series of slender and tapering rods forming an acute angle with the central axis. This fringe is the vexillum or "vane." The central axis is divisible into two distinct parts—a hollow, cylindrical, transparent calamus, or "quill," the base of which is inserted into the skin, and a solid, quadrangular rhachis or "shaft" supporting the vane. At the lower end of the quill is a small hole—the lower umbilicus—through which the nutritive pulp passes during the growth of the feather, while at the upper end, where it passes into the shaft, a similar hole will be found the upper umbilicus—and from this the last remains of the cap sules which contained the nutritive pulp may sometimes be seen protruding. If the quill is cut open a series of these capsules will be found fitting one into the other throughout the length of the chamber.
The rods composing the vane are the rami or "barbs," and will be found, on microscopic examination, to be lath-shaped and to taper to a point. Each barb supports a double series of radii, or "barbules." These "barbules" differ markedly in structure on the two sides of the barb, those pointing towards the tip of the feather—the "anterior barbules"—being ribbon-shaped from the base outwards for about half their length, when they become cut up to form a series of long, delicate hooklets (fig. I). On the opposite side of the barb the barbules are also ribbon-shaped for about half their length, but the ribbon is curved trough-fashion, so that the whole series of posterior barbules forms a number of deep valleys, and into these the booklets are thrust so as to catch hold of the upper edges of the troughs, which are set so that the upper edge is towards the upper, and the lower towards the under surface of the feather (fig. I).
In one of the primary or "quill" feathers of the wing of a crane, each barb of the inner side of the vane was found to bear about 600 pairs of barbules, which would make about 800,000 barbules for the inner web of the vane alone, or more than a million for the whole feather. It is to these booklets that the closely-knit elastic vanes of the flight feathers and the body feathers are due. Where these are wanting the barbs do not adhere, resulting in a loose "discontinuous" vane such as is found in the plumes of the ostrich.
Many feathers, in addition to the main axis, bear a second, generally much shorter axis, supporting a loose discontinuous vane ; this is the "aftershaft" and arises from the under surface of the feather. In the cassowary and emu the aftershaft is as large as the main shaft.
There are several different kinds of feathers—contour feathers, semiplumes, down-feathers, filoplumes and powder-down. Contour feathers are those which form the outline of the body, and are all that can generally be seen. Those which form the "flight feathers" of the wing, and the tail feathers, are the most per fectly developed. Semiplumes are degenerate contour feathers. The down-feathers are generally hidden by the contour feathers: they form in many birds, such as gulls and ducks, a thick under clothing comparable to the under-fur of seals. In all cases they are loose, soft and "fluffy," the barbs being long and slender, while the barbules are of ten long and provided with knob-like thickenings answering to the hooklets of the contour feathers; these thickenings help to "felt" the separate down-feathers to gether, the barbs of one down-feather interlocking with those of its neighbour. Down-feathers do not possess a main axis, all the barbs arising from a common centre. Filoplumes are degenerate structures having a superficial resemblance to hairs, but they always bear a minute vane at the tip. They occur in all birds, in clusters about the bases of contour feathers. In some birds they may project beyond the contour feathers, forming con spicuous white patches, as on the thighs of cormorants. In their early stages of development they often possess a large aftershaft. The eyelashes and bristles round the mouth in many birds appear akin to filoplumes. Powder-down feathers are degenerate down feathers which secrete a dry, waxy powder. This rapidly disinte grates and becomes distributed over the plumage adding thereto a peculiar bloom. In birds of the heron tribe powder-down feathers have reached a high degree of development, forming large patches on the breast and thighs.
Nestling Down.—The majority of young birds either emerge from the egg clothed in down-feathers, or develop these a day or two afterwards. But this covering, though superficially similar in all, may differ widely in its constitution, even in closely re lated forms, while only in a few species can the complete history of these feathers be made out.
The tawny owl (Strix aluco) is one of these. At hatching, the young is thickly clad in white. woolly down-feathers, of the character known as umbelliform—that is to say, the central axis or main shaft is wanting, so that the barbs all start from a common centre. These feathers occupy the position of the ultimate con tour feathers. They are shortly replaced by a second down-like covering, superficially resembling, and generally regarded as, con tour-feathers but of a "semi-plumous" type. They differ from the down-feathers which preceded them in that their barbs spring from a central axis as in typical contour feathers. Feathers of this last description indeed have made their appearance in the shape of the "flight" or quill feathers (remiges) and of the tail feathers. This plumage is worn until the autumn, when the adult plumage is assumed. The down-feathers which appear at hatch ing are known as pre-pennae when they precede contour feathers, or pre-plumulae when they precede down feathers. The first generation of pre-pennae, in the tawny owl is made up of pro toptyles, while the succeeding plumage is made of mesoptyles, and these in turn give place to the teleoptyles or adult feathers. The two forms of nestling plumage—pre-pennae and pre-plumulae —may be collectively called "neossoptyles." In the nestling cormorant the down represents pre-plumulae only; in hawks a mixture of pre-pennae and pre-plumulae ; in owls pre-pennae only.
As a rule the nestling develops but one generation of neossop tyles, and this generally answers to the mesoptyle plumage.
As development proceeds and the contour feathers make their appearance they thrust the mesoptyle feathers out of their fol licles—the pockets in the skin in which they were rooted—and these will often be found adhering to the tips of the contour feathers for many weeks after the bird has left the nest.
The Colours of Feathers.—In variety and brilliancy of colour birds are not surpassed by any other group of animals. Yet the pigments to which these colours are due are few in num ber, as many resplendent hues are produced by structural peculiari ties of the colourless horny surface of the feathers.
The principal colour pigments are (a) melanin pigments, de rived possibly from the haemoglobin of the blood, or from the blood plasma, and (b) lipochrome or "fat" pigments, which are regarded as reserve products; though in birds this is doubtful.
The melanin pigments (zoomnelanin) occur as granules and give rise to black, brown and grey tones; or they may combine with the lipochrome series.
The lipochrome pigments (zoonerythrin and zooxanthin) tend to be diffused throughout the substance of the feather, and give rise respectively to red and yellow colours.
Turacin, found in the quill-feathers of touracoes and elsewhere, is a reddish-purple pigment, containing 5 to 8% of copper. These feathers lose their colour when wet, but regain it on drying.
What effect food may have on colour in birds in a wild state we do not know, but it is significant that flamingoes and linnets in confinement never regain their bright hues following their first moult in captivity. If cayenne pepper be mixed with the food of certain strains of canaries, from the time the birds are hatched onwards, the yellow colour of the feathers becomes in tensified, till it takes on a deep orange hue. Bullfinches, if fed on hemp-seed, turn black.
Structural colours include all metallic or prismatic colours, blue, green, white, some yellows, and, in part, glossy black. In metallic feathers the barbules are modified in various ways, fre quently to form flattened, overlapping plates or tiles, while the surfaces of the plates are either smooth, finely striated or pitted. But, save in white feathers, beneath this colourless, glazed outer coat there is a layer of pigment.
While in many birds the coloration is a uniform hue or in bands and patches of colour on the breast more or less brilliant, in others it is sombre, and made up of dark longitudinal stripes or transverse bars on a lighter ground. The latter is the more primi tive, and there seems good reason to believe that longitudinal stripes preceded transverse bars. This is indicated by the fact that the nestlings of the more primitive groups are longitudinally striped, and that young hawks in their first feather-plumage are so striped, while the adults are barred.
There is also evidence that the evolution of brilliant plumage began with the males, and has, in many cases, been acquired by the females, and lastly by the young, as in the kingfishers. Often where the parents are alike the young wear a different, duller livery, as in the common starling (Sturnus vulgaris) . But where the female differs from the male in coloration the young resemble the female. The physiological explanation of complete disappear ance of pigment in adult life, e.g., the gannet, is not apparent.
The "quill" feathers of the wing and tail are renewed in pairs, so that flight is little impaired, the change taking place in the wing from the wrist inwards, as to the primaries, and from the body outwards, towards the tip of the wing, as to the secondaries. In certain birds, however, as in ducks and rails, all the quill feathers of the wing are shed at once, so that for some time flight is impossible.
In the penguins this simultaneous moulting is carried still further. The old feathers covering the body are replaced en masse.
Some birds moult twice within the year, the additional moult taking place in the spring, as in the warblers (Sylviidae) . But when this is the case the spring moult is only partial, since the quill feathers of the wings and the tail feathers are not renewed.
At this spring moult a special "nuptial" plumage, commonly conspicuous for its vivid coloration, is often assumed, e.g., god wits, knots, dunlins, and ruffs. This plumage, at first assumed at the mating period by the males only, and doffed soon after the young appear, has been retained in some species for longer and longer periods, so that the succeeding plumage is worn only for a few weeks, as in many ducks, wherein the males, as soon as the young are hatched, assume an "eclipse" dress.
But the assumption at the breeding season of a brilliant plum age is not always due to a moult. In many birds, notably many passerines, this change is brought about by shedding the tips of the feathers, which are duller than the rest. In this way the bright rose pink of the linnet's breast, the blue and black head of the chaffinch, and the black throat and chestnut-and-black markings of the back of the sparrow, are assumed.
These last form a single super ficial layer of flattened cells— the e the cells of the malpighian layer, which are cylindrical in shape and form several layers. The papillae assume a cone-shape with its apex directed back wards, while the base is car ried down by the growth of the malpighian cells, so that the cone is sunk in a deep pit.
Thereby these malpighian cells become divided into two portions: (I) those taking part in the formation of the walls of the "feather follicle," (2) those enclosed within the cone. These last surround the central mass or core formed by the dermis. This mass constitutes the nutritive pulp for the development of the growing feather, and is highly vascular. The cells of the malpighian layer within the cone now become differentiated into three layers: (I) an inner, extremely thin, forming a delicate sheath for the pulp, and found in the fully developed feather in the form of a series of hollow, trans parent caps enclosed within the calamus ; (2) a thick layer which forms the feather itself ; and (3) a thin layer which forms the investing sheath of the feather. As growth proceeds the cells of this middle layer arrange themselves in longitudinal rows to form the barbs, while the barbules are formed by a secondary splitting. At their bases these rudimentary barbs meet to form the ohachis. Finally the tips of the barbs break through the investing sheath and the fully formed feather emerges. A part of the pulp and malpighian cells remains over after the growth of the two genera tions of down-feathers, and from this, succeeding generations of adult feathers are developed. The reproductive power of the feather follicle appears almost inexhaustible, since it is not di minished appreciably by age, or restricted to definite moulting periods.
The feathers of birds grow only along certain definite tracts known as pterylae, leaving bare spaces or apteria. These pterylae differ in their conformation in different groups of birds, and hence are of service in systematic ornithology.
The principal pterylae are as follows: — ( I.) The head tract (pt. capitis) , which embraces the head only.
(2.) The spinal tract (pt. spinals), which extends the whole length of the vertical column. It is one of the most variable in its modifications. In its simplest form it runs down the back in the form of a band of almost uniform width, but generally it expands considerably in the lumbar region, as in Passeres. Frequently it is divided into two portions; an upper, terminating in the region of the middle of the back in a fork, and a lower, which commences either as a fork, e.g., plover, or as a median band, e.g., swallow.
(3.) The ventral tract (pt. ventralis), which presents almost as many variations as the spinal tract. In its simplest form it runs from the throat backwards in the form of a median band as far as the base of the neck, where it divides, sending a branch to each side of the breast. This branch commonly again divides into a short, broad outer branch which lodges the "flank" feathers, and a long, narrow, inner branch which runs backwards to join its fellow of the opposite side in front of the cloacal aperture. This branch lodges the abdominal feathers. For convenience sake the cervical portions of the spinal and ventral tracts are generally regarded as separate tracts, the pt. colli dorsalis and pt. colli ventralis respectively.
(4.) The humeral tract (pt. humeralis) , which gives rise to the "scapular" feathers.
(5.) The femoral tract (pt. femoralis), which forms an oblique band across the thigh.
(6.) The crural tract (pt. cruralis) , which clothes the rest of the leg.
(7.) The tail tract (pt. cacudalis) , including the tail feathers and their coverts ; and (8.) The wing tract (pt. alaris), which presents many peculiar fea tures. Each segment—arm, forearm and hand—bears feathers essential to flight, and these are divided into remiges, or "quill" feathers, and tectrices, or "coverts." The remiges of the arm, collectively known as the parapteron and hypopteron, are composed respectively of long, quill-like feathers forming a double series, the former arranged along the upper, and the latter along the lower aspect of the humerus. They fill up the gap which, in long-winged birds, would otherwise occur during flight between the quill-feathers of the forearm and the body. In short-winged birds they are reduced. The quills of the forearm are known as "secondaries," those of the hand as "pri maries." The former are attached to the ulna by their bases at relatively wide distances apart, while the primaries are crowded close together and attached to the skeleton of the hand. Birds which fly much have the outer primaries of great length, giving the wing a pointed shape, as in swifts, while in species which fly little the outer primaries are short, giving the wing a rounded appearance.
The tectrices or wing coverts are arranged in several series, decreasing in size from behind forwards. The number of rows on the dorsal aspect and the method of their overlap afford characters of general importance in classification.
The wings of struthious birds differ from those of the Neogna thae in many ways. All are degenerate and useless as organs of flight. Those of the ostrich and rhea are the least degraded.
In the ostrich ankylosis has prevented the flexion of the hand at the wrist joint so that the quills—primaries and secondaries— form an unbroken series of about 4o in number. Of these 16 belong to the primary or metacarpo-digital series, a number ex ceeding that of any other bird. The coverts in their disposition, bear a general resemblance to those of neognathine wings; but they differ on account of the length of the feathers and the absence of overlap.
The wing of the rhea more nearly resembles that of flying birds since the hand can be flexed at the wrist joint, and the primaries are 12 in number, as in grebes. The coverts, as in the ostrich, are remarkable for their length. In both ostrich and rhea, as well as in all the other struthious birds, the under surface of the wing is entirely bare. The wing of the cassowary, emu and apteryx has undergone complete degeneration, so that only a vestige of the hand remains. The wing of the penguins has become trans formed into a paddle, clothed on both sides with a covering of small, close-set feathers. A pollex is wanting, as in the cassowary, emu and apteryx.
Commercial Applications of Feathers.—The chief pur poses for which feathers become commercially valuable may be comprehended under four divisions :—(1) Bed and upholstery feathers; (2) quills for writing; (3) ornamental feathers; and (4) miscellaneous uses.
Bed and Upholstery Feathers.—The qualities which render feathers available for stuffing beds, cushions, etc., are lightness, elasticity, freedom from matting and softness. These are com bined in the most satisfactory degree in the feathers of the goose and allied aquatic birds. Goose feathers and down, when plucked in spring from the living bird, are most esteemed. The down of the eider duck, Somateria molissima, is valued above all other substances for lightness, softness and elasticity; but it has some tendency to mat, and is consequently more used for quilts and in articles of clothing than unmixed for stuffing beds. The feathers of swans, ducks and the domestic fowl are also largely employed for beds ; but in the latter bird, the feathers are harsher and less downy than are those of the natatorial birds generally.
Quills for Writing.—The earliest period at which the use of quill feathers for writing is recorded is the 6th century ; and from that time till the introduction of steel pens early in the 19th century they formed the principal writing implements of civilized communities. It has always been from the goose that quills have been chiefly obtained, although the swan, crow, eagle, owl, hawk and turkey all have been laid under contribution. Swan quills, indeed are better than are those from the goose, and for fine lines crow quills have been much employed. Only the five outer wing feathers of the goose are useful for writing, and of these the second and third are the best, while left-wing quills are more esteemed than those of the right as they curve outward and away from the writer using them. Quills obtained in spring from living birds are the best.
Ornamental Feathers.—Feathers do not appear to have been much used in Europe for ornamental purposes till the close of the 13th century; during Elizabeth's reign feathers began to occupy an important place as head-dress ornaments of women. Ostrich feathers hold a pre-eminent position among ornamental feathers ; and the ostrich is the only bird reared exclusively for the sake of its feathers. Ostrich farming is one of the established industries of South Africa, and is also practised in North Africa, Argentina, Arizona and California. The feathers are cut from the living animal and the stumps withdrawn later. In the male, the long feathers of the tail, and wings are white, and the short feathers of the body are jet black; while the tail and wing feathers of the female are white, tinged with a dusky grey. The feathers of the male are consequently much more valuable than those of the female, and are separately classified in commerce. The art of the plumassier embraces the cleaning, bleaching, dye ing, curling and making up of ostrich and other plumes and feathers.
In addition to those of the ostrich, the feathers of certain other birds form articles of steady commercial demand. Among these are the feathers of the South American ostrich, Rhea americana, the marabout feathers of India obtained from Leptoptilus argala and L. javanica, the aigrettes of the heron, the feathers of the various species of birds of paradise, and of numerous species of humming-birds.
Miscellaneous Applications of Feathers.—Quills of various sizes are extensively employed as holders for the sable and camel hair brushes used by artists, etc. Feather brushes and dusters are made from the wing-feathers of the domestic fowl and other birds; those of a superior quality, under the name of vulture dusters, being really made of American ostrich feathers. A minor application of feathers is found in the dressing of artificial fly hooks for fishing. (W. P. P.)