The New International Encyclopædia/Bird

BIRD (origin obscure). Birds are warm-blooded vertebrate animals clothed with feathers, and normally capable of flight. They comprise the class Aves. They have existed since early Mesozoic times, exhibit an interesting evolution within very compact and well-defined limits, dwell in every part of the globe, are exceedingly diverse in size, form, ability, and appearance, adapted to every sort of climate and food, show mental qualities of a high order, are of great importance in their economic relations with man, and in most cases are beautiful in outline and color and possessed of melodious voices.

. Birds are classified between the reptiles, regarded as inferior to them, and the mammals, regarded as superior in general organization. Birds differ from mammals, broadly, in being clothed with feathers instead of hairs, and in the absence of milk-glands, and by sundry differences in anatomy and methods of existence, such as the hatching externally of eggs, and the devotion of the fore limbs to flight. They differ from reptiles in having a covering of feathers instead of scales; a complete double circulation of warm blood; no more than three digits in the manus, long legs, etc.

Affinity with Reptiles.—The differences last noted are, however, of much less importance than those which separate them from mammals, and the structural resemblance is so close that some anatomists, notably Huxley, have included reptiles and birds in a single group, the Sauropsida, comparable to Fishes or Mammals, and completing, with them, the three divisions of the Vertebrata. This grouping was founded upon the fact that birds and reptiles were alike in being oviparous or ovoviviparous; in having a cloaca; in the incompleteness of the diaphragm, and of a corpus callosum in the brain; in having only one occipital condyle; in the presence of a movable quadrate bone and other peculiarities of the skull; and in the fact that the ankle-joint is between two sets of tarsal bones. The close relationship thus implied has been confirmed by the disclosures of paleontology, which show that birds have a reptilian ancestry, and are an off-shoot of the same stock as modern reptiles.

Development of the Class.—For details of the geological evidence of the origin and evolution of birds, the reader is referred to the articles here to sum up the matter as Prof. A. Newton, following Fürbringer, does in his Dictionary of Birds (London, 1893-96). Birds, since they spring from reptiles, must have begun with toothed forms of small or moderate size, with long tails and four lizard-like feet, having well-formed claws, while their bodies were clothed with a very primitive sort of down. To them succeeded forms wherein down developed into feathers, and the fore and hind limbs differed in build—the former becoming organs of prehension (as is still the case in some young birds), and the latter the chief instruments of progression. Then followed a dinosaur-like transformation of the pelvis and legs, and a gradual coalescence of the ankle-bones, enabling birds more and more to walk erect. These early reptilian birds were flightless and terrestrial, or at most climbed trees. “Among those which possessed this habit, the befeathering (which as yet had, like the hair of mammals, served only for
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warmth) presumably entered upon a higher step, the feathers becoming larger on certain parts of the body, particularly on the fore limbs and tail, so as to begin to act as a parachute and allow of a safe gliding descent from a height. By successive increase in the stiffness and size of the feathers, and corresponding modification and strengthening of the skeleton and muscles, the possibility of incipient but real flight was afforded.” Thus far this explanation of primitive bird-formation is theoretical; but at this point comes confirmatory evidence in the fossil remains of ../Archæopteryx/ (q.v.)—an actual bird with real feathers and apparently considerable power of flight, that lived at the beginning of Secondary or Mesozoic time. “The faculty of flight, thus acquired, went on improving. The remiges grew stronger and stronger . . . and in proportion as the fore limbs specialized into highly developed wings. . . the tail shortened and was consolidated, the posterior vertebræ becoming united as a pygostyle. Thus originated. . . the higher or better ‘Birds of Flight.’ This type was established in the Cretaceous Ichthyornis, and includes the vast majority of existing birds commonly grouped as Carinatæ (i.e. with a keeled breastbone—not Ratitæ); but these only in later times developed their various higher modifications, which were rendered possible by the saving of material and weight.” It is important, however, to bear in mind that all birds did not reach the highest degree of faculty in flight, “Many stopped, as it were, half way,” to continue Newton's summary of Fürbringer's conclusions, “when a retrogression of the power already attained took place; or, if the power were reached, it could not be maintained—an easy life and absence of rivalry inducing an increased bulk of the body, until the utmost exertion of muscular strength could no longer sustain it in the air. Thus when this retrograde movement began, occasion was afforded for the dwindling away of the volant power, and hence arose the different types which are commonly grouped as Ratitæ (ostriches, etc.).”

. Though a strictly scientific arrangement might call for the more basal internal structure to be first considered, it suits present purposes better to give first an account of the exterior—the plumage and epidermal appendages so characteristic of birds.

Plumage.—Feathers are horny products of the epidermal cells of the skin, but very different from mammalian hairs or reptilian scales. Scales, in the herpetological sense, are pieces of the skin itself, originating as folds of the cuticle; and hairs arise from involuted pockets in the skin; whereas feathers are produced from papillæ of the cuticle. They grow so as to form a coat over the whole body, called plumage. (For the structure and characteristics of a feather considered alone, see ; for the peculiarities of their growth in ‘tracts,’ see renewal of plumage, see .) The origin of plumage can only he surmised. It may well have been that chance variation which marked the earliest divergence from a reptilian stock toward the bird type. It has been conjectured that the primitive plumage was in the form of down, and the probability of this is shown in the fact that at present infant birds are first clothed
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with down alone. In a large class of birds, mostly of low organization, this down is assumed before the embryo leaves the egg; such are called Dasypædes, and as most of them are sufficiently advanced to begin at once to run about and pick up a living without parental help, they are said to be pre-cocial, and their parents classed as Præcoces. Among the higher birds, as a rule, however, the young are quite naked when hatched, and are termed Gymnopædes (or Psilopædes), and these birds are spoken of as altricial (Altrices), because they must feed and care for the young.

Origin and Hervice of Plumage.—The first and essential service of the feathers is as a coat, protecting the body against cold and atmospheric changes. In the view of Prof. O. C. Marsh, who did more than any one else to collect and reason upon the facts of the origin and development of birds, the acquirement and gradual increase of the feather coat was accompanied by a steadily augmented warmth of blood, and a proportional increase in its activity. The greater activity would result in a more perfect circulation, and this in a steady improvement of general bodily ability and power. If this view be correct, to the acquirement of plumage may be attributed the high degree of development attained in modern birds as a class. The second great service of plumage is its development, on wings and tail, into an instrument of flight. (For the phenomena of flying, see .)

Wings and Their Functions.—Wings, externally viewed, are formed of large quill or flight feathers (remiges), which grow from the posterior margin of the arms and hands in a plane (when outstretched) horizontal with the long axis of the body; they overlie one another at the outer edge so as to slide much beneath one another when the wing is closed, form an aeroplane when distended and quiet, and a series of oars when the bird moves. It is believed that in the development of the wings (Gadow, Proceedings Zoölogical Society of London, 1888, page 665) and powers of flight in birds, those feathers nearest the body were the first to become serviceable; but now the most important remiges are those near the outer margin of the wing. These form a set (usually ten, but in some groups nine, and in a few cases eleven), which spring from the bones of the hand and are called primaries; they are long and comparatively narrow, and the shaft is near to the anterior margin. Inside these, springing from the ulua, is a set of lesser and more equally proportioned quills called secondaries, or better, cubitals; these are variable in number, the humming-bird having only six, while the albatross has forty. The fleshy parts of the wing are covered with rows of feathers called coverts, which gradually diminish into the general ‘scapular’ plumage upon and overlying the shoulder. Wing-feathers vary in length, strength, and shape, according to the mode of life and power of flight in different birds; and there is consequently great variety among birds in respect to the relative size, shape, and power of the wings themselves. The extreme of length, both absolutely and relatively, is found among the oceanic birds, whose lives may be said to be spent upon the wing. The albatross and the condor have wings two and one-half times the total length of the body, reaching an alar expanse of between 11 and 12 feet in large

specimens, and nearly the same proportions hold for other pelagic wanderers, as the tropic-birds; while in such birds as swifts and hummers 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, endurance, 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 ‘whittled away’ on the inner vane toward the tip. From this extreme, wings grade down in shape 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 secondaries 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 prolonged sustention or swift darting; yet they do not lack power, for many short-winged birds make journeys of hundreds of miles without resting (see ), 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 expected that birds which, for any reason, diminished 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 probability 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 almost wingless condition, now most extreme in the apteryx (see ), is due to degeneration of these organs, following the acquirement of cursorial habits in treeless regions. The same result came about differently in another direction, 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 metamorphosed 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 degeneration 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 barren islands. The great auk or gare-fowl (Plautus impennis), 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 flightless. Several other instances might be adduced; and the fact that the dodo, dinornis, and other extinct flightless birds were inhabitants of islands, strengthens the thesis. See .

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 ) 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 protection. Spread above the nest, they shed rain and hot sunshine from the eggs or tender fledglings. 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, chaha (see ), 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 expression of the emotions, especially in the nuptial season, wings are utilized in many ways by these highly emotional animals. They are curiously 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 some 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 manakins. 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 s) 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. Frequently, however, there are present other sexual characters, more or less transitory, in the form of elongated plumes, as in the argus pheasant, the standard-wing (see Plate of s), 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 ). Lastly, wings seem to offer the privacy and darkness grateful to tired nature, 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 purposes. It may be invisible, as in the apteryx, a 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 rectrices, the insertion of which is concealed by under and upper coverts, either of which may

sometimes be transformed into highly ornamental plumes. The tail-feathers grow in pairs, the reason for which arrangement is shown by Archæopteryx, where each vertebra of the lizard-like tail supported one on each side. The reduction of this long caudal skeleton to a pygostyle, and the consequent crowding of the feathers upon a short base, seems to have produced the modern fan-shaped tail. “The normal number of rectrices is 6 pairs, but a few birds possess 10 or 11; several 9, 8, 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 Passeres the utmost variety exists, the magnificent lyre-bird (see Plate of, 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 symmetrical beauty of the lyre-bird's tail; and of the amazing caudal ornaments of birds of paradise. (See Plates of s; s; .) “The motmot,” remarks F. M. Chapman (see, for interesting details), “gesticulates with its tail in a remarkable manner, 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 reminds one of a band-master flourishing his baton. We 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 be also expressive of disposition. Compare the drooped tail of a pensive fly-catcher with the uplifted member of an inquisitive wren.”

Crests, Combs, etc.—Crests in very great variety, 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 ) of herons, and the ‘tassels’ hanging from the turkey's breast, are developments of the plumage that might properly be discussed here; but the limitations of space compel a reference instead to the biographies of the various birds that exhibit them. The beak and head-appendages, such as combs, wattles, carunculated ceres, ‘helmets,’ etc., are, however, dermal growths, which require mention; as also will the coverings of the feet.

The Beak.—The beak, bill, or neb of a bird is formed of two horny sheaths, overlying bony supports, and forming the mouth and food-getting organ of a bird. It consists of two mandibles, an upper and a lower, into which the foremost bones of the skull are produced, all appearance of lips being lost. It is not furnished with proper teeth, although rudiments of them have been observed in the embryos of some parrots; and the marginal laminæ with which the bills of many water-fowl are furnished partake of the same character, being secreted by distinct pulps. The resemblance of these marginal laminæ to teeth is particularly marked in the merganser (q.v.). The bills of birds differ much, according to their different habits, and particularly according to the kind of food on which they are destined to live, and the manner in which they are to seek it. In birds of prey, the beak is strong; the upper mandible arched or hooked, and very sharp; the edges sharp, often notched, and the whole beak adapted for seizing animals, and tearing and cutting to pieces their flesh. A powerful, short, hooked beak, sharp-edged and notched, indicates the greatest courage and adaptation to preying on living animals. The beak of the vulture is longer and weaker than that of the eagle or falcon. In birds which feed on insects and vegetable substances, the hooked form of the beak is less common, though well marked in parrots; those birds which catch insects on the wing, such as the nightjars, swifts, etc., are remarkable for the deep division of the beak, and their consequently wide gape, and an analogous provision to facilitate the taking of prey is to be observed in herons, kingfishers, and other fishing birds; but the object is attained in their case by the elongation of the beak, whereas birds which catch insects on the wing have the beak very short. Birds which feed chiefly on seeds have the beak short and strong, for bruising them; while the beak of insectivorous birds is comparatively slender.

Many aquatic birds have broad, comparatively soft and sensitive bills, with laminæ on the inner margin for straining the mud from which much of their food is to be extracted; other birds, as snipes, avocets, etc., seeking their food also in mud, have slender bills of remarkable sensibility. As the varieties of form roughly distinguished the larger and more obvious of the groups of birds, the bill was made the standard for the early classifications, giving such group-names as Dentirostres, Conirostres, etc., no longer of scientific value. The modifications of form are very numerous, and the peculiarities of the bills of toucans, hornbills, spoonbills, crossbills, parrots, humming-birds, etc., are very interesting, and intimately connected with habits. Some of the modifications, however, are sexual and transitory, such as the knob on the beak of the pelican, and the deciduous parts of that of

the puffin. At the base of the upper mandible, a portion of the beak is covered with a membrane, called the cere (Lat. cera, wax, from the waxy appearance which it presents in some falcons, etc.), which in many birds is naked, in others is feathered, and frequently is covered with hairs or bristles. The nostrils are situated in the upper mandible, usually in the cere, but in some birds they are comparatively far forward, and in some, as puffins, they are very small and placed so near the edge of the mandible as not to be easily detected. They are more or less open, or covered with membrane, or protected by hairs or feathers. Besides their principal use for seizing and dividing or triturating food, the bills of birds are employed in a variety of functions, as dressing or preening the feathers, constructing nests, etc. They are also the principal instruments used by birds in their combats.

The Feet.—The feet of birds vary considerably, according to their mode of life. In some the claws are strong and hooked, in others short, straight, and weak; in some the toes are all separate, in others more or less connected; in birds especially adapted for swimming they are generally webbed or united by a membrane; in other swimming birds, however, a membrane extends only along the sides of each toe. A large group of birds, including woodpeckers, parrots, etc., have two toes before, opposed by two behind, the foot being thus particularly adapted to grasping irregular surfaces and facilitating climbing. These distinctions were seized upon by the early ornithologists as a means of classification, which gave us such obsolete group-names as Scansores, Rasores, etc., which only partly coincide with the more scientific groupings now accepted. In most birds the tibia is feathered to the heel-joint; in some, however, and particularly in waders, the lower part of it is bare; the shank and toes are generally destitute of feathers, and are covered with scaly skin, and the arrangement of these scales has been much studied. “The most primitive form of the horny covering of the feet,” according to Stejneger, “seems to be its division into uniform hexagonal scales, and is called reticulate; the next stage is when some of these scales fuse together, forming what are termed scuta, or scutella;. . . still further specialization is indicated by the tarsal scuta fusing into a continuous covering, which, in its extreme development, embraces both the front and the back of the tarsus, as in some of the higher groups of passerine birds; such a tarsus is said to be ‘booted.’ ” Feet, like beaks, have certain accessory or changeable parts; thus in the grouse family there is a seasonal molting of the sheaths of the claws. The most important appendage, however, is the spur or series of spurs, which arm the ‘heels’ of certain gallinaceous cock birds, and form serviceable weapons among this class, whose beaks are ill adapted to use as weapons, so that they strike with feet and wings in their combats. The extraordinary development of the legs and feet in ratite birds will be found described under , , and similar titles.

Oil-gland.—The skin of birds has no sudoriferous nor sebaceous glands, but on the rump at the base of the tail (on the ‘pope's nose’) is a bilobed gland containing oily matter, which is present in all birds except the Ratitæ, bustards. and a few others. This secretion seems useful

only for cleaning the plumage, and the bird squeezes a little out upon its beak before preening its feathers. In the oil-bird the secretion is excessive, and that of the hoopoe has a very disagreeable odor.

. Skeleton.—The theory and facts of ancestry related above are enforced by comparative anatomy, which finds in the structure of birds a close resemblance to the internal structure of reptiles. Birds, nevertheless, possess distinctive anatomical features fully entitling them to rank as a separate class. In general form the body is spindle-shaped, tapering forward from the shoulders to the head, with the feathers sloping backward, and again diminishing toward the tail; this is adaptive to progress through the thin media of air and water inhabited by birds; and the weight is thrown forward, securing proper balance for the normally horizontal position. In order to secure this there is a concentration of muscles and other organs in the region below the point of suspension of the wings. Not only are the fleshy portions of the legs mainly confined to the upper portions of these limbs, but the muscles which elevate the wings are actually placed on the under instead of the upper surface of the body.

Though the neck is sometimes much prolonged, the body is very compact and rigid. The backbone consists of vertebræ having (typically) saddle-shaped, articular facets, and many processes and ligaments which lock them firmly together; moreover, the vertebræ of the back generally become ankylosed or firmly united together by cementing bone, the solidity thus acquired being for the support of the ribs, and these also are proportionately stronger than is usual in mammals; each of them is provided in the middle with a flattened bony process, directed obliquely backward to the next rib, so that they support one another, and they often become ossified with the sternum, giving unusual rigidity and strength to the thoracic framework. The hinder part of the spine is consolidated with the pelvis, and the tail, primitively long, has become shortened into a few small movable vertebræ terminating in a short and generally much elevated bone, consisting of ankylosed vertebræ, called the pygostyle, or plowshare bone.

In contrast to the general stiffness of the vertebral column in the trunk, it is remarkable for great fiexibility in the neck, enabling a bird to make ready use of its bill, or to bring its head into such positions as suit the adjustment of the centre of gravity in flying, standing, etc.; there are also certain peculiar ligamentous bands, by which birds can retain the neck in the customary S-curve without muscular exertion. The first (atlas) and second (axis) vertebræ are modified to form an articulation with the head, which is completed by a single globular condyle (as in reptiles), forming a sort of pivot, and enabling the head to be turned around with a freedom and to an extent impossible to the mammalia; and the fact that it is formed almost wholly by the basioccipital bone constitutes a fundamental distinction between birds and the reptiles on one side and the mammals on the other.

The Skull.—The skull is formed of bones corresponding with those of man; but they can be distinguished only when the bird is very young, soon becoming consolidated together. The braincase “is more arched and spacious, and is larger,

in proportion to the face, than in any reptilia” except pterodactyls. The jaws are much elongated, so as to form the bill, the outward shape of which, however, formed by the horny sheath, may be very different from that of its bony supports, in adaptation to habits of use. The upper mandible is formed anteriorly and mainly of the præmaxillæ, and posteriorly by the paired maxillaries and other bones, varying greatly in relative position and importance, and in many birds movable, so that the upper jaw is capable of a certain amount of up and down motion as on a hinge. These and other elements form a palate, the varying bony arrangement of which has been used with disputed success as a basis for general classification in ornithology. The lower jaw is formed of several elements now fused solidly together; and it is connected with the skull by the quadrates and other lesser bones and by a series of elastic cartilages permitting extreme movability and large expansion of the gape.

Legs.—The limbs of birds conform to the vertebrate type, with certain modifications, which are least in the hinder pair. They are attached to the ‘pelvic arch,’ which in birds consists of some solidified sacral and coccygeal vertebræ and the paired pelvic bones (ilium, ischium, and os pubis), which latter meet to form a cup in which

rests the head of the femur. When one considers how far behind the centre of gravity of a bird's body the hip-joint is placed, and how it must sustain the whole weight of the body under unfavorable circumstances, as well as provide for the leverage of the muscles of the thigh, the necessity for this consolidation, breadth, and massiveness will be perceived. The thigh is short, and concealed within the body; the next division, often mistaken for the thigh, is the leg, strictly so called, or tibia and fibula, which ends at what is really the ankle-joint, although popularly regarded as the knee; and beneath this is the shank, commonly called the tarsus, which in some birds is very long, serving as a part not of the foot, but of the leg, and formed by a compound bone, composed of the united metatarsal bones, with the upper end of which four tarsal bones are fused. Thus the ankle-joint is not between the tarsus and metatarsus, but between two series of tarsal bones. The tibia is therefore properly a tibiotarsus, and the so-called tarsus is really a tarsometatarsus. The feet are divided into toes, which are four in number, three before and one behind, differing from each other in length and in the number of joints or phalanges of which they are composed, the hallux, which is directed backward, being in general comparatively short, and consisting of two joints only. No fifth toe is known, and the hallux (first toe) is absent in the Struthiones, which have lost also the second, while the fourth is diminished, throwing the work almost alone upon the third, or middle toe.

Wings and Shoulder-Girdle.—The anterior limbs or wings of a bird are the fore limbs of other vertebrates modified for the purposes of flight. They are attached to the shoulder-girdle or pectoral arch of the skeleton, which in this, class is of extraordinary strength and importance. It consists of the thoracic case formed by the dorsal vertebræ, the true ribs and the breast-bones (sternum), which are connected by ligaments and ossifications. All the bones of birds are dense and hard. The sternum is remarkably large and strong, serving for the attachment of the muscles which depress the wings. It generally exhibits a projecting ridge, or keel, along the middle, which is proportionately largest in birds of most powerful flight or swimming ability, and is wanting only in the flightless birds (see above). This plate ossifies only as the bird matures, being in early life a cartilage, the flexibility of which may easily be felt in young fowls, affording housekeepers a test for alleged ‘spring’ chickens. The variations in form of the sternum are of considerable taxonomic significance. Outside and attached to this case

on each side, at a point as high and forward of the centre of gravity as feasible; is the framework of the shoulder-joint, braced against the tremendous leverage of the wings; it consists of the scapula, or shoulder-blade, a coracoid and a clavicle, the junction of which forms a cup in which rests the head of the humerus. The scapula is a more or less sabre-shaped bone placed above the ribs, and lying parallel to the spine, imbedded in muscles and firmly attached (sometimes fused) at the forward extremity to the head of the coracoid. The coracoid is a pillar-like bone which serves as the main support of the shoulder, “for while at one extremity it sustains the wing, at the opposite end it is firmly united to the front of the sternum. A third brace is afforded by the combined clavicles, which curve downward and backward from the shoulder-joint, imbedded in muscles, and unite under the neck, and just in advance of the sternum, to form the furculum (fork), which we know as the ‘wish-bone’, and the English call ‘merry-thought.’ ” See.

The wing-bones are homologous with those of the arm in other vertebrates, but have become modified in both form and relative position to adapt them to the requirements of flight. “The fore limb of a bird, when in a state of rest,” says Huxley, “exhibits a great change of position if it be compared with that of an ordinary reptile; and the change is of a character similar to, but in some respects greater than, that which the arm of a man presents, when compared with the fore limb of a quadruped mammal. The humerus lies parallel with the axis of the body, its proper ventral surface looking outward. The forearm is in a position midway between pronation and supination, and the manus is bent back upon the ulnar side of the forearm, in a position not of flexion, but of adduction.”

The humerus is usually short as compared with the radius and ulna (the latter is more curved and stouter than the former) which together form the forearm and main stretch of the visible wing; these remain separate, and at their outer extremities articulate with the two small bones of the wrist (carpus) which alone remain free. The other, distal, carpal bones “fuse in the embryo with the proximal end of the three first metacarpals, and all trace of their originally separate existence disappears.” The metacarpus consists of three bones (I., II., III.), which are more or less flattened and fused. The first bears the pollex or ‘thumb’ (whence springs, in some birds, the bastard wing or alula), and the second the index finger, which is long, straight, and has two, or sometimes three, phalanges. These bones complete the wing, except that a few species have a rudimentary third finger. Archæopteryx, however, had three well-developed fingers, all armed with claws; and in embryo ostriches a claw sometimes appears on the third finger. Such is the bony framework of the wings, operated by muscles of immense power (the pectoral muscles are said to make about one-fourteenth of the weight of the whole body in birds of prey, and one-eleventh in a wild goose); but their usefulness depends upon the external furniture of feathers, heretofore described, which forms the expanse necessary for buoyancy and flight (see following paragraph).

. The muscles of birds conform, in general, with the vertebrate type, though the homologies, especially about the fore limbs, are sometimes obscure. For these homologies, Newton's review in Bronn's Thier-Reich (Vögel, pp. 9-325) is complete, and is epitomized in the former's Dictionary of Birds, pp. 602-620. Specified arrangements of muscles and tendons, especially the plantar tendons, have been much experimented with as a basis of classification, but the results have not proved of great value when taken alone. Only a few general considerations need be presented here. The muscular system of birds is characterized by an extraordinary strength in those parts concerned with activity and endurance, and has certain interesting automatic functions. Thus the vast pectoral muscles attached by a broad fleshy base to the sternum and narrowing to a complicated tendinous attachment to the wing-bones, are mainly two, forming the thick whitish ‘breast’ of edible fowls; they depress the wing—the effective movement in flight. An elastic band in front of the wing passes from the head of the humerus to the carpal joint; when the wing is extended it is stretched about three times its ordinary length, and its natural contraction enables the bird to keep the wing closed without effort. As it lies just beneath the skin, it is easily cut, and as such a severance prevents the wing from being firmly extended, it offers the best method of rendering captive birds incapable of flight; to disable one wing is quite enough. See .

“In the hind limb of most birds there is a singular extensor muscle, which arises from the pubis, ends in a tendon which passes to the outer side of the knee-joint, and terminates in the leg by uniting with the flexor digitorum perforatus. The result of this arrangement is, that the toes are flexed whenever the leg is bent upon the thigh, and, consequently, the roosting bird is held fast upon his perch by the weight of his own body.” (Huxley, Anatomy of Vertebrated Animals, New York, 1878.)

. The digestive apparatus of birds resembles that of mammals; exhibiting, however, various modifications, according to the different kinds of food—some birds feeding on flesh, others on fish, others exclusively on insects, others on seeds, others more indiscriminately on a variety of animal and vegetable substances. Few birds masticate their food in any degree; upon being swallowed, it enters the crop or craw, an enlargement of the œsophagus situated just before the breast-bone, where it is moistened by saliva and partly digested, so that its contents, easily regurgitated, form a suitable food for nestlings in some families, as, notably, the pigeons. The crop is wanting in the ostrich, and also generally in birds that feed on fish, and is of greatest size in those of which the food consists of seeds or grain. It is generally single, and on one side of the gullet; sometimes, as in pigeons, it is double. A second dilatation of the œsophagus, called the proventriculus, is generally largest in those birds in which the crop is wanting or small; and in this the food is further softened and chemically treated. The third and principal stomach is the gizzard (q.v.), which in birds of prey, fish-eating birds, etc., is a mere menbranous sac, but in birds which feed on grain or seeds is very thick and muscular.

so that it acts as a sort of mill, and with extraordinary power. In these birds, also, the grinding down of the contents of the gizzard is assisted by the small rough pebbles or grains of sand constantly swallowed, as is well exemplified in domestic fowls. The liver and pancreas are usually large, as also are the kidneys; but there is no urinary bladder, and the urine, which contains very little water, is at once poured into the cloaca, an enlargement of the terminal portion of the intestine. The intestine varies greatly in capacity and length, being ‘short’ in all purely frugivorous and insectivorous birds, and ‘long’ in those which live upon fishes, carrion, grain, and grass; its walls differ in structure from the mammalian and agree with the reptilian type. The diaphragm is incomplete and differently arranged from that of mammals. See .

Food.—The amount of food required to sustain the high temperature and great bodily activity of birds is excessive, when compared with that required by other animals of proportionate size; and young, growing birds tax the food-getting abilities of their parents to the utmost. Many facts might be quoted in support of this, such as that of Aughey's confined plovers, which ate an average of 200 locusts and similar large insects each day; fiedglings habitually consume more than their own weight of food between sunrise and sunset—sometimes twice as much. Adult birds eat almost continuously, and digestion is extremely rapid, the process being completed in from one to two hours in small birds. This ability for enormous consumption makes the birds of vast value to man in the reduction of insect pests; it also limits the time birds may fast, so that unless they can fly with very great rapidity, long journeys cannot be undertaken, and the fact that most birds are able to make great speed is probably an achievement of natural selection related to this very characteristic. On the other hand, birds show considerable ability to adopt a new diet, and adapt themselves to it healthfully. The writings of Dr. F. E. L. Beal, Prof. S. A. Forbes, and others, published in the documents of the United States Department of Agriculture, and by the governments of Illinois and other States, contain much information on this subject.

Respiration in Birds is rapid and copious, and the pulmonary system includes not only a pair of rather large lungs, but a system of ‘air-sacs,’ by which air is distributed throughout the body. These air-sacs are of two kinds. One consists of membranous expansions of the lungs, which lie among the tissues in the form of inflatable bags controlled by muscles, so that they can be emptied and filled at the will of the bird; they occur in all parts of the body and penetrate beneath the skin and inside many bones, but in varying amount. It is popularly believed that all bird-bones are hollow, but this is far from true. “Generally, the skeleton is most pneumatic in large birds that fly well, like vultures, storks, swans, and pelicans; less so in small birds, and least in heavy or little-flying water-birds. However, there are many exceptions. While, for instance, most of the bones of many Passeres, of swifts, divers, rails, the kiwi, and of terns, are solid, and air-cells are restricted chiefly to the cranium, many parts of the skeleton of the large Ratitæ are very

The greatest development of pneumatic cells exists in the screamers and hornbills, in which even the fingers and toes, in fact, any part of the skeleton, are hollow. . . . It is well known that a bird which has its humerus shattered by shot can for some time breathe, although its beak and nostrils be tightly closed, and thus be submitted to unnecessary, excruciating pain. Compression of the thorax and abdomen suffocates a wounded bird better than strangulation.” See .

The functions of these air-sacs are not very evident. They serve to ventilate the rather incontractable lungs, and from them issues the air which inflates the ‘drums’ in the neck of the prairie chicken, the gular pouches of the adjutant stork, the ‘horn’ of the bell-bird (q.v.). and that extraordinary swelling of the throat which marks excitement in the pectoral sandpiper (see Colored Plate of ). These inflations are characteristics of male birds only, and appear only in the spring. The air-sacs also assist in furnishing a continuous current of air enabling some singing birds to maintain a long strain of melody, such as the nightingales.

Arterial Circulation.—Conformably with this copious aëration, and the habits of activity, the circulation of the blood in birds is rapid, the heart beating 120 times a minute at rest, and nearly doubling this rate at the first stroke of the wings; and its temperature (from 100° to 112° F.) is considerably higher than in mammals. The heart resembles that of the mammalia in its form and structure; but the right ventricle, instead of a mere membranous valve, is furnished with a strong muscle to impel the blood with greater force into the lungs; and the carotid arteries are peculiar. During incubation there is a congestion of blood in the enormously dilated vessels of the abdominal wall, forming the ‘brood-organ.’ The red corpuscles are, on the average, twice as large as those of man, and elongated, as in reptiles, rather than round, as in most mammals. See .

Reproductive System.—Birds are distinctly male and female, cock and hen. The females possess a pair of ovaries (of which only the left one is functional as a rule) situated in the ‘small of the back’ at the front end of the kidney. The ovary consists of a grape-like cluster of germinal eggs, which during the breeding season exhibit all stages of size and ripeness, but in winter are reabsorbed, so that the determination of sex by dissection at that season is difficult. The ripe egg passes down tubular passages (oviducts) in which it receives, in certain ‘uterine’ expansions, its coverings of albumen and shell, to the right side of the rectum, and thence is voided in a more or less advanced state of development, which is completed by incubation or its equivalent. The testes of the male are a pair of glands, situated as are the ovaries, from which the spermatic ducts lead to the cloaca. (See and .) All birds lay eggs, some only one, others as many as twenty, the number seeming to depend upon the average chances of reaching sexual maturity possessed by each species. With rare exceptions, these eggs are cared for by the parents, and all birds of higher organization prepare receptacles (nests) for them, and furnish by brooding the continuous

warmth necessary to their hatching—a process which requires from two to six weeks according to size and other factors. This necessity has developed a body of instincts, habits, and mental and structural characteristics, which include the most striking, significant, and interesting facts of bird life and history. See.

. The nervous system of birds is highly developed, but presents few salient differences from the vertebrate type. There is a perceptible improvement of the brain over that of reptiles, more especially in the increased proportional development of the cerebral hemispheres; but the surface-convolutions and other features that play so important a part in the brains of the higher mammals are absent or merely indicated. In this, and in the way the cerebral nerves originate, the brain remains nearly reptilian. The sympathetic system is highly developed, as would be expected in creatures of such activity and sensitiveness. See .

The Senses of Birds, as a whole, equal those of most mammals, and in some directions are superior to those of other animals. Taste is, perhaps, feeblest, the horny nature of the jaws and the character of the food not favoring the

of this sense in birds; yet a certain amount of discrimination is undoubtedly exercised by them, apart from the guidance of touch and smell. Odors affect birds also, but to a less degree probably than is frequent among even reptiles, not to say mammals; and it is probable that even the carrion-eating species depend little upon their sense of smell in guiding them even to odoriferous food. Birds owe less to the sense of touch than many other creatures, for they have few tactile organs, the bristles about their beaks having other purposes, mainly. The bills of some mud-hunting species, however, are specialized in this direction (see and ). In respect to hearing, birds are well endowed, having the internal apparatus of the ear (see ) highly perfected, and exposed to the vibrations of the air through a cartilaginous meatus with a fairly wide opening. One function of the ears of birds, as well as of other creatures, seems to be to inform the animal as to its relations with space, balance, direction, etc. Whether or not this is to be regarded as a sense, is undecided. There is no external ear, the nearest approach being in the nocturnal owls (where the organ presents other peculiarities—see ), though the so-called ‘ears’ of some of those birds are merely tufts of feathers which have nothing to do with audition. Moreover, this faculty in birds extends beyond mere quick perception of sounds to ‘the power of distinguishing or understanding’ pitch, notes and melodies, or music; and it has been an important factor in the development of bird-singing.

The vocal performances of birds are one of their foremost and most pleasing characteristics; yet of only a few, in the most highly organized classes, can it be said that they sing. Even in the order called ‘singers’ (Oscines), many make little melody, though, like the crow, they may utter a long list of significant notes—a real language, which is developed among birds generally to a degree realized by few. The vocal

of this class is not in the larynx, but at the lower or pectoral end of the windpipe, in an organ called the syrinx, which consists of several stretched and vibratory membranes, tracheal rings and controlling muscles. This complicated organ, peculiar to birds, presents almost unlimited modifications, and is quite unlike the arrangement of laryngeal vocal cords in mammals, for the tongue has nothing to do with a bird's speech or music. (See Plate, Figs. 7, 8, 14.) Further consideration of the singing of birds leads into the domain of their evolution.

The Eyes of Birds are large, clear, highly perfected in structure, and, in many species, endowed with a power of vision which is beyond anything known elsewhere. The way in which vultures descend upon carrion from a height where they are invisible to us, or sea-birds swoop down upon fishes, or fly-catchers dart long distances upon insects so small we can barely perceive them at close range, are examples of their sharp-sightedness, with which is combined remarkable ability in rapidly changing the focus, as must be necessary to a bird darting swiftly upon prey it must keep constantly in view. There are no movable eyelids, but in place of them a nictitating membrane is frequently drawn over the eyeball to clean and lubricate it, or shield it from a glare. The owls and other nocturnal birds have great power of contractility

not only in the pupil, but in the whole case of the eye. See.

A &lsquo;Sense&rsquo; of Direction.—The so-called ‘sixth sense’ of orientation or direction, that is, the power of instinctively knowing and maintaining a course of flight through the air independent of

guidance of landmarks, is manifested by birds in a most surprising way; for no other explanation (little as we understand it) seems to account for their ability to cross vast spaces of ocean straight toward small islands or restricted areas of coasts, as some birds habitually do in their migrations. See.

. Many aspects of bird-life properly belonging to a treatise more extended than this need only be indicated here, because they are elsewhere discussed with reasonable fullness. Thus their classification is outlined under. The distribution of birds on the globe, where, in spite of their powers of flight, most groups and species are strictly bounded in their range, is sketched under . The seasonal movements that characterize most birds outside the tropics, leading them to make more or less extended journeys, along the customary routes, northward in the spring and southward again in the fall, will be found explained under. The singing of birds, which forms so conspicuous and enjoyable a feature of their life, is treated elsewhere in this article, and their nest architecture, eggs, care of young, and domestic traits, are treated under ; ornaments, weapons, economic value, and habits in general, are portrayed in the biographies to be looked for under the names of various species and groups, and in such articles as ; like. Birds have also taken a part in the history of mankind's mental and religious development, as appears in the articles ; and. Finally, many species have been exterminated, directly or indirectly, since mankind came upon the earth, an account of which will be found under.
 * etc.; while their peculiar colors,
 * and the
 * and the

. General Works.—Newton, A Dictionary of Birds (London, 1896); Evans, Cambridge Natural History: Vol. IX., “Birds” (Cambridge, 1900); Stejneger, Standard Natural History, Vol. IV., “Birds” (Boston, 1885); Sharpe, Royal Natural History, Vols. III., IV., “Birds” (London, 1898); Huxley, “On the Classification of Birds” in ''Proc. Zoöl. Soc. of London'' (London, 1867; and ib., 1868, pp. 294-319); Nitsch and Burmeister, System der Pterylographie (Halle, 1840), translated into English by Dallas, and published by the Royal Society (London, 1867), as Pterylography; Fürbringer, Untersuchungen zur Morphologie und Systematik der Vögel (Amsterdam, 1888).

Faunal Works, North America.—Wilson, American Ornithology (Philadelphia, 1810-14); several more recent editions, of which a good one is Brewer's (Boston, 1853, reprinted, New York, 1853), and the finest is that issued by Porter & Coates (Philadelphia, 1871); Audubon, Birds of America (London, 1827-38); Swainson and Richardson, Fauna Boreali Americana, Part Second, Birds (London, 1831); Audubon, Ornithological Biography (Edinburgh, 1831-39); Nuttall, Manual of the Ornithology of the United States and Canada (Boston, 1832-34); DeKay, Zoölogy of New York, Part II., “Birds” (Albany, 1844); Baird, Pacific Railroad Reports, Vol. IX. (Washington, 1858), reprinted with an atlas of 100 plates by Baird and Cassin as Birds of North America (Philadelphia, 1860); Baird, Brewer and Ridgway, History of North

Birds, “Land Birds” (Boston, 1875); Water Birds (Cambridge, 1878); Chapman, Bird Life (New York, 1898, edition in colors); Coues, Key to North American Birds (Boston, 3d ed., 1887); Ridgway, Manual of North American Birds (Phihidelphia, 1887); Chapman, Handbook of Birds of Eastern North America (New York, 1895) (the three last named are technically descriptive only); Coues, Birds of the Northwest (Washington, 1874); Coues, Birds of the Colorado Valley (Washington, 1878; contains an extensive bibliography from 1612 to 1878); Cooper, Ornithology of California (San Francisco, 1870); Nelson, Natural History Collections in Alaska (Washington, 1887); Turner, Contributions to the Natural History of Alaska (Washington, 1886); Warren, Birds of Pennsylvania (Harrisburg, 1890); and many popular illustrated works.

Europe.—Temminek, Manuel d'ornithologie (Paris, 1815 and 1820); Brehm, Naturgeschichte der Vögel Deutschlands, etc. (Ilmenau, 1831); Gould, Birds of Europe (London, 1832-37); Naumann, Naturgeschichte der Vögel Deutschlands, new edition edited by Blasius (Leipzig, 1860); Fritsch, Naturgeschichte der Vögel Europas (Berlin, 1871); Dresser, Birds of Europe (London, 1879), 8 vols. quarto, &ldquo;unquestionably the most complete work of its kind, both for fuHness of information and beauty of illustration.&rdquo; For British birds in elaborately illustrated works, see Yarrell, History of British Birds (last edition, edited by Saunders, London, 1885); McGillivray, History of British Birds (London, 1837-40); Bree, History of Birds of Europe (London, 1875-76); Morris, History of British Birds (London, 1858); Seebohm, History of British Birds and Their Eggs (London, 1883-85); for a more condensed history, see Montagu, Ornithological Dictionary, Newman's edition (London, 1883).

Asia, Japan, and Oceania.—Pallas, Zoögraphica Russo-Asiatica (Saint Petersburg, 1811); Gould, Birds of Asia, completed by Sharpe (London, 1883); David and Oustalet, Les oiseaux de la Chine (Paris, 1877); Taczanowsky, “Faune Ornithologique de la Sibérie Orientale,” in ''Memoirs Acad. of Sciences'' (Saint Petersburg, 1891); Blanford, Zoölogy of Persia (London, 1876); Seebohm, Birds of Asia (London, 1901); Jerdon, Birds of India (Calcutta, 1862); Blanford, Fauna of British India: Birds (London, 1893); Blythe, Mammals and Birds of Burma (London, 1875); Oates, Birds of British Burmah (London, 1883); Legge, Birds of Ceylon (London, 1880); Hume, Nests and Eggs of Indian Birds (Oates's edition, London, 1890); Blakiston and Preyer, Birds of Japan (London, 1882); Seebohm, Birds of the Japanese Empire (London, 1890); Gould, Birds of Australia (London, 1848, supplements, 1869)—magnificent folios of colored plates; Gould, Handbook to the Birds of Australia (London, 1865)—reprint, with additions, of the text of the foregoing; Buller, Birds of New Zealand (London, 1888); Wallace, Malay Archipelago (New York, 1869); Gould, Birds of New Guinea, completed by Sharpe (London, 1881); Hartlaub and Finsch, Beitrag zur Fauna Centralpolynesiens (Halle, 1867); Salvadori, Ornitologia della Papaasia e delle Mollucche (Turin, 1882; enlarged, 1891); Wilson and Evans, Birds of the Sandwich Islands (London, 1884).

For the Philippines, see Von Martens, Journal für Ornithologie (Leipzig, 1866); Lord Walden,

''Trans. Zoöl. Soc. of London'' (London, 1844); ''Proc. Zoöl. Soc. London (London, 1878 and 1879, many articles); Wardlaw-Ramsay, Ibis (London, 1884 and 1886); Blasius, Trans. Linnæan Soc.'' of London, and Ibis (London, 1884-88); and ''Proc. Zoöl. Soc.'' (London, 1888, and subsequently).

Africa.—Layard, Birds of South Africa, 2d ed. (London, 1884); Andersson, Notes on the Birds of Damara-land (London, 1872); Heuglin, Ornithologie nordostafrikas (Cassel, 1875); Finsch and Hartlaub, Vögel Ostafrikas (Leipzig, 1870); Hartlaub, System der OrnitholoGie West Africas (Bremen, 1857); Shelley, Handbook to the Birds of Egypt (London, 1872); Tristram, Fauna and Flora of Palestine (London, 1884); Milne-Edwards and Grandidier, ''hist. naT. des'' oiseaux de Madagascar (Paris, 1884).

South and Central America.—Azara, Hist. ''nat. de los pájaros del Paraguay y Rio de la Plata'' (Madrid, 1805); Belt, A Naturalist in Nicaragua (London, 1888); Bates, The Naturalist on the Amazon (London, 1863); Sclater and Salvin, Biologia Centrali Americana, Birds (London, ( 1880); Cory, Birds of the West Indies (Boston, 1889); Darwin, A Naturalist's Voyage (London, 1860); Sclater and Hudson, Argentine Ornithology (London, 1889). See also Proceedings of the United States National Museum, and those of the Zoölogical Society of London, since 1870.