Degeneration of Wings and Flight-Power.— There is, then, a close relation between the form of wings and the habits of their owner, so far as these involve flight. Hence it might be ex pected that birds which, for any reason, dimin ished or were prevented the use of their wings would find these gradually reduced in utility. This, as a matter of fact, has happened. As has already been intimated, there is every probabil ity that the ratite birds (ostriches, cassowaries, and various forms recently extinct) began to be affected in this feature early in the history of the class, and that their present flightless and al most wingless condition, now most. extreme in the apteryx (see Kiwi), is due to degeneration of these organs, following the acquirement of cursorial habits in treeless regions. The same result came about differently in another diree tion, where the penguins, developing by untoward circumstances of competition in the Antarctic region, and forced to get their food wholly from the sea, have had their wings utterly meta mo•phosed from flight-organs. into swimming organs. The related auks of the Arctic regions have continued to find use for their wings, and have retained them in fair condition; but these include an example of how this kind of degenera tion of an organ may come about in a species whose home is so restricted and peculiar (perhaps in freedom from natural enemies) that little or no call is made for exercise of the wings—such circumstances as might surround a bird or breed of birds on any of many small and nearly bar ren islands. The great auk or gare-fowl (Plautus impcnnis), although somewhat more free than in the case supposed, seems to have degenerated to its wingless condition in such a way; and another, more pertinent example is the weka (q.v.) of Tristan d'Acunha, which, otherwise little different from a gallinule, is entirely flight less. Several other instances might be adduced; and the fact that the dodo, dinornis, and other extinct flightless birds were inhabitants of is lands, strengthens the thesis. See FLIGHTLESS BIRDS.
Secondary Uses of Wings.—Wings have other uses than for flight. They seem to have been organs for climbing before they were organs for flying, and immature birds of several sorts (see HOACTZIN ) still use them in scrambling about branches or through the reeds or over rough ground. They enable many water-birds to dive and swim, by motions under water analogous to flying. They form hiding-places and shelters for the young. "As a hen gathereth her chickens under her wings," was an expression chosen in Holy Writ to enforce the idea of supreme pro tection. Spread above the nest, they shed rain and hot sunshine from the eggs or tender fledg lings. They serve as weapons also, for many birds strike with them very effectively, as our barnyard turkeys show; and in certain forms, as the jacana, •hah• (see SCREAMER), and the spur-winged goose, the outer edge of the wing is furnished with one or more spurs which are highly effective as weapons. As a means of ex pression of the emotions, especially in the nup tial season, wings are utilized in many ways by these highly emotional animals. They are curi ously developed, and are fluttered. elevated or spread, or beaten against the body or upon the air, or clapped together over the back, to make drumming, or crackling sounds indicative of excitement, and serving as signals or challenges. In some cases certain parts of the wings are modified, as is the case in the snipe (q.v.), where sonic of the uttermost primaries are shaped so as to make a distinctively audible noise in flight (when desired) known as bleating; and .still more strangely in the case of certain mana kins. In color, wings are likely to be plain—at least so far as the quill-feathers are concerned. The proudly displayed wing-colors of the sun bittern (see Plate of TirsTAnos) is one of a Jew exceptions. Ordinarily there are bars of and in the case of the ducks a brilliant metallic 'speculum,' and the linings of wings are often more beautiful than the outside. Fre quently, however, there are present other sexual characters, more or less transitory, in the form it elongated plumes, as in the argils pheasant, the standa•d-wing (see Plate of Nin I TJA RS and some of the birds of paradise (q.v.). These belong to the male alone. and may or may not be permanent. Their explanation and history comes under the domain of 'sexual selection' (see NAT URAL SELECTION ) . Lastly, wings seem to offer the privacy and darkness grateful to tired N a ture. for a bird desiring to sleep usually 'tucks
its head under its wing.' The Tail.—The tail, like the wings, presents great variety as to form and serves sundry pur poses. it may be invisible, as in the apteryx, it downy tuft, as in the dodo, very short, broad and stiff, or very long, flexible and narrow, plain and simple or highly ornate. Like the wings, it is composed of large quill-feathers called rye trices, the insertion of which is concealed by under and upper coverts, either of which may sometimes be transformed into highly ornamen tal plumes. The tail-feathers grow in pairs, the reason for which arrangement is shown by Ar clneopteryx, where each vertebra of the lizard like tail supported one on each side. The re duction of this long caudal skeleton to a pygo •tyle, and the consequent crowding of the feathers upon a short base, seems to have pro duced the modern fan-shaped tail. "The normal number of rectrices is 6 pairs, but a few birds possess 10 or 11: several 9, S, or 7; many only 5, and Crotophaga (anis) only 4—the diminution being brought about by the suppression of the outer pair or pairs." The main purpose of the tail, which is capable of much movement, is to assist the bird to balance, check, and guide itself. There seems, however, to be no relation between the size and shape of the tail and the character of the wings, though its shape is pretty constant within each order. Thus most of the sea-birds, water-fowl, waders. and game-birds have short, compact, inconspicuous tails, until we come to the pheasants. where they are likely to be exceedingly long and showy. The pigeons and their allies have broad tails, and those of the birds of prey are stout and square. The picarian families are mostly possessed of tails. which are long, or strikingly colored, as notably in the trogons and some cuckoos, or marked with peculiarities of form as in the motmots, or are the subject of special ornamentation as among the humming-birds. In the great tribe of Pas seres the utmost variety exists, the magnificent lyre-bird (see Plate of LYRE-BIRD, ETC.) coming next to the bob-tailed pittas; and here, among the short-tailed finches. warblers, and thrushes, are found birds capable of such caudal displays as those of the paradise-birds. sunbirds, some jays. etc. This shows that the essential service of the tail in flight may be performed by an appendage of almost any shape, and that the -conspicuous, flexible, and somewhat unimportant nature of the organ has caused it to be influenced by natural (and especially by sexual) selection in an unusual degree as a means of displaying sexual ornamentation and emotion. This is the explanation of the possession by male pheasants of their magnificent trains, of the gorgeous array spread by the peacock (though here it is really the tail-coverts). which he rattles nervously, as if to call attention to the show; of the symmetri cal beauty of the lyre-bird's tail; and of the amazing caudal ornaments of birds of paradise. (See Plates of PEACOCKS; LYREBIRDS: BIRDS OF PARADISE.) "The motmot," remarks F. M. Chapman (see SloruroT, for interesting details), "gesticulates with its tail in a remarkable man ner, swinging it from side to side, so that it suggests the pendulum of a clock, or sweeping it about in circles with a movement which re mind: one of a band-master flourishing his baton. 'Xe shall find in other species, also, that the tail, more than any other organ, is used to express emotion. 'Recall its twitching and wagging: how it is nervously spread or 'jetted,' showing the white outer feathers, as in the meadow-lark. The tail may he also expressive of disposition. Com pare the drooped tail of a pensive fly-catcher with the uplifted member of an inquisitive wren." Crests, Combs, rte.—Crests in very great va riety, usually able to be elevated or depressed at the will of the bird, and ornamental plumes or tufts of feathers, such as the aigrets (see AIGRET) of herons, and the 'tassels' hanging from the turkey's breast, are developments of the plu mage that might properly be discussed here; but the limitations of space compel a referenee in stead to the biographies of the various birds that exhibit them. The beak and head-appendages, such as eoliths, wattles, earuneiilated eeres, 'hel mets,' etc., are, however, dermal growths, which require mention: as also will the coverings of the feet.