BIRD. 96 iJlKD. specimens, and nearly the same proportions liold for other pelagic wanderers, as the tropic-birds; while in such birds as swifts and Immnicrs the wing may largely exceed the body-length. Such wings belong to birds which range over vast spaces of ocean, or remain poised for long periods in the upper air, as do the vultures and diurnal hawks, or that feed upon flying insects, and so have need of extraordinary sustaining power, en- durance, and ability for occasional extreme speed and agility. Such wings are always slender, and sabre-like in outline, when extended, the primaries being usually much longer than the secondaries, narrow and curved, but very strong and far overlapped; those of the diurnal birds of prey are peculiarly narrowed, or 'whit- tled awaj on the inner vane toward the tip. From this extreme, wings grade down in sliajie to the 'round' or 'short' type characterizing the terrestrial and arboreal birds generally, or those which seek their food in limited areas of quiet water. Here the wing may be considerably shorter than the body, and its feathers broad, the primaries little, if at all, exceeding the sec- ondaries in length — all broadly and evenly vaned, so that the breadth of the wing approaches its length. Such are adapted to quick starting and short, agile 'flitting' flights, rather than to pro- longed sustention or swift darting; yet they do not lack power, for many short-winged birds make journeys of hundreds of miles without rest- ing (see Migration), and others are capable of extremely swift motion ; but none of them can soar, as do sea-birds and birds of prey, nor equal these in volant grace. All these, moreover, trust for safety rather to hiding than to fleeing from danger. 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 diirerently in another direc- tion, where the penguins, developing by untoward circumstances of com|)etition in the Antarctic region, and forced to get their food wholly from the sea, have had their wings utterly meta- morphosed 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 conie about in a species whose home is .so restricted and peculiar (jierhaps 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 (I'lautus impetmis) , 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 dillerent from a gallinule. is entirely lliglit- less. Several other instances might l>c adduced; and the fact that the dodo, dinornis. and otiicr 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 Ho.CTZiN) 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-i^laces and shelters (or 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, chahi (see Sckeamek), and the spur-winged goose, the outer edge of the wing is furnished with one or more spurs which are highly etrective 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 ujion the air, or clapped together over the hack, to make drunuiiing 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 some of the uttermost primaries are shaped so as to make a distinctively audil)le 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 lie plain — at least so far as the quill-feathers are concerned. The proudly displayed wing-colors of the sun- bittern (see Plate of Bu.stards) is one of a few exceptions. Ordinarily there are bars of white, 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 arc present other sexual characters, more or less transitory, in the form of elongated plumes, as in the argus |)heasant, the standard-wing (see Plate of Niohtjahs), and some of the birds of paradise (q.v.). These belong to the male alone, and may or may not be ])ermanent. Their explanation and history comes luider the domain of 'sexual selection' (see Xat- IRAL Selection). Lastly, wings seem to olfer the privacy and darkness grateful to tired na- 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 jiur- poses. It may be invisible, as in the ajjleryx. a downy tuft, as in the dodo, very short, broad and stifl", or very long, flexible and narrow, ])lain and simple or highly ornate. Like the wings, it is compo.sed of large quill-feathers called rec- trices, the insertion of which is concealed by under and upper coverts, either of which may
