EXOSKELETON.

The epidermal exoskeleton of birds is very greatly developed, feathers constituting its most important part.

Three kinds of feathers are found, viz. (a) pennae including quills and coverts, (b) down feathers or plumulae, and (c) filoplumes which are rudimentary feathers. The structure of the different kinds of feathers is described on pp. 303-306.

Sometimes a fourth class of feathers, the semiplumae, is recognised. They have the stems of pennae, and the downy barbs and barbules of plumulae.

In most birds the pennae are not uniformly distributed over the whole surface of the body, but are confined to certain tracts, the pterylae; while the intervening spaces or apteria are either bare or covered only with down feathers. In some birds, however, such as the Ratitae and the Penguins, pennae are evenly distributed over the whole body.

In many birds the calamus or quill bears two vexilla or vanes, the second of which, called the aftershaft or hyporachis, is generally much the smaller, and is attached to the under surface of the main vexillum. In the Moas, Emeu and Cassowary the two vexilla in the adult bird are nearly equal in size; though in the nestling Emeu one is much longer than the other. The aftershaft is very small in most Passeres and gallinaceous birds, but is comparatively large in Parrots, Gulls, Herons and most birds of prey. It is absent or extremely small in the Ostrich, Apteryx, Rhea, Pigeons, Owls, Anseres, and others.

The quill feathers include two groups, the remiges or wing quills, and the rectrices or tail quills. In most birds the primary remiges, or those which are attached to the bones of the manus, are ten or eleven in number, and are set in grooves in the bones, being firmly attached to them. In the Ostrich however the primaries are little specialised in character and are as many as sixteen in number. They are also less definitely attached to the bones; as their ends do not lie in grooves in the bones, but project beyond them.

The secondary quills or those attached to the ulna vary much in number according to the length of the bone. The large dark quills in the wings of Cassowaries are the secondaries.

The wing of Penguins is very little differentiated. It is covered at the margin by overlapping scales which gradually merge into scale-like feathers at the proximal end. The wing of the Penguin has nothing comparable to the remiges of other birds.

In some birds, such as Herons (Ardea), there occur in places plumulae of a peculiar kind, which grow persistently and whose summits break off into fine powder as fast as they are formed. These feathers are known as powder-down feathers. They occur also in some Parrots and are then scattered indiscriminately all over the body.

Other exoskeletal structures besides feathers are commonly well developed. Thus the extremities of the jaws are sheathed in horny beaks whose form varies enormously according to the special mode of life.

In ducks and geese the beak with the exception of the anterior end is soft, and its edges are raised into lamellae, while in the Mergansers these lamellae become pointed processes supported by bony outgrowths. These lamellae act as strainers. In Parrots and Hawks, on the other hand, nearly the whole of the beak is hard.

The toes and tarso-metatarsus are usually featherless and are covered either with granular structures or with well-formed scales. The toes are nearly always provided with claws, and these vary in correlation with the character of the beak. Claws[105] also sometimes occur on the manus. Thus Archaeopteryx and some Ostriches and Rheas have claws on all three digits. Most Ostriches and Rheas, and many Anseres and birds of prey, have them on the first two digits, while the Secretary bird (Gypogeranus) and many fowls, ducks, and birds of prey, especially kestrels, have a claw only on the pollex. In the Cassowary, Emeu, Apteryx and some Ostriches and Rheas only the second digit is clawed.

Claws should not be confounded with spurs, which are conical horny structures developed on bony outgrowths of the radial side of the carpus, metacarpus, or metatarsus. They occur in a number of birds, but are most commonly developed in gallinaceous birds, by which they are used for fighting. A single spur occurs on the metacarpus in Megapodius, in Palamedea, in Parra jacana and in Hoplopterus spinosus, the Spur-winged plover. The Derbian Screamer, Chauna derbiana, has two metacarpal spurs, borne on the first and second metacarpals. The Spur-winged goose, Plectropterus gambensis, has a carpal spur borne on the radial carpal. Metatarsal spurs are quite common.

The male Solitaire (Pezophaps) has large bony excrescences on the wrist which may, like spurs, have been sheathed in horn and used for fighting.

Teeth do not occur in any living birds, but conical teeth imbedded in separate sockets are present in Archaeopteryx and Ichthyornis, while in Hesperornis similar teeth occur implanted in continuous grooves in the mandibles and maxillae, the premaxillae being toothless.

Except that teeth are partly dermal in origin, a dermal exoskeleton is quite unrepresented in birds.

ENDOSKELETON.

Perhaps the most striking feature of the endoskeleton of birds is its pneumaticity. In the embryo all the bones contain marrow, but as growth proceeds this becomes replaced by air to a variable extent in different forms. In all birds some part of the skeleton is pneumatic. Many small birds and Apteryx and Penguins among larger ones have air only in the skull; in Pigeons air is present in all the bones except the caudal vertebrae, the leg bones, and those of the antibrachium and manus; in Hornbills every bone contains air.

Vertebral Column.

The vertebral column of birds is readily divisible into a very mobile cervical region, and an extremely rigid post-cervical region. In most birds the vertebral centra are without terminal epiphyses, but these structures are found in Parrots. The cervical vertebrae are generally large and vary in number from eight or nine to twenty-three in Swans. Except in some extinct forms, such as Ichthyornis and Apatornis, in which they are biconcave, the centra are characterised by having saddle-shaped articulating surfaces, which in front are concave from side to side and slightly convex from above downwards, while posteriorly they are convex from side to side and concave from above downwards. The atlas is small and ring-like, and its centrum is fused with the axis forming the odontoid process. Cervical ribs are often well developed, and in some of the Ratitae they remain for a long time distinct from the vertebrae.

The thoracic vertebrae are distinguished from the cervical by the fact that their true ribs are united to the sternum by means of sternal ribs. This distinction, however, though convenient, is somewhat arbitrary, as it has been shown that in the fowl and gannet, two pairs of ribs which in the adult are free from the sternum, are connected with it in the embryo. When, as in the Swans, the thoracic vertebrae are not all fused together, they generally have saddle-shaped articulating surfaces, but sometimes, as in the Penguins, Auks and Plovers, the centra are convex in front and concave behind. The trunk vertebrae generally have well-marked neural spines, while in the Divers the anterior ones have peculiar bifurcating hypapophyses.

The trunk vertebrae are not readily divisible into thoracic and lumbar. There are two true sacral vertebrae, but as development proceeds a number of other vertebrae become fused with the true sacrals, the whole forming a large compound sacrum. These pseudosacral vertebrae generally include the lumbar, and some of the thoracic and caudal vertebrae. Sixteen to twenty vertebrae or even more may be included in the compound sacrum, and sometimes the whole of the trunk vertebrae are fused together. In Archaeopteryx however but five vertebrae take part in the formation of the sacrum.

In Archaeopteryx there are twenty long caudal vertebrae, of which the last sixteen carry a pair of feathers apiece, but in all other birds the tail is short and in the great majority of cases the posterior vertebrae are fused together, forming the pygostyle. In the Ratitae and Tinamidae a pygostyle is rarely or imperfectly developed. In Hesperornis there are twelve caudal vertebrae, six or seven of which are united by their centra only, forming an imperfect pygostyle.

The free caudal vertebrae are generally amphicoelous.

The Skull.

The skull of all birds from Archaeopteryx onwards is essentially similar, differing from the skull of reptiles mainly in the extent to which the cranium is arched, and its greater size in proportion to the jaws.

Most of the bones of the cranium are pneumatic, and all show a marked tendency to fuse together, and have their outlines obliterated by the disappearance of the sutures. The several bones remain longest distinguishable in the Ratitae and to a less extent in the Penguins. The orbits are very large and lie almost entirely in front of the cranium; they are separated by an interorbital septum which is sometimes, as in Chauna and Scythrops, very complete, sometimes, as in Hornbills and the Common Heron, very slightly developed. As a general rule the sclerotic is cartilaginous.

The anterior nares are almost always situated far back at the base of the beak near the orbits, but in Apteryx they are placed right at its extremity. In Phororhacos they are placed very high up on the enormous beak and are not separated by any bony partition.

The skull of Parrots has some peculiarities. In some Parrots the lachrymal sends back a process which meets the postorbital process of the frontal and completes the orbit. In most birds the upper beak is immovably fixed, but in some it is attached to the cranium, only by the nasals and by flexible processes of the premaxillae, so that by this means a kind of elastic joint is established and the beak is able to be moved on the cranium. In the Parrots and Opisthocomus there is a regular highly movable joint.

In Cassowaries the fronto-nasal region of the skull is produced into an enormous bony crest, and in Hornbills a somewhat similar structure occurs. Although true teeth do not occur in any known bird except Archaeopteryx, Hesperornis, and Ichthyornis, another extinct bird, Odontopteryx, has the margins of both jaws provided with forwardly-directed tooth-like serrations, formed of part of the actual jawbone: a living hawk, Harpagus, too, has a deeply notched bill, to which correspond serrations in the premaxillae.

A basi-pterygoid process of the basisphenoid abuts against the pterygoid in Ratitae and in Tinamous, plovers, fowls, pigeons, ducks and geese among Carinatae, recalling the arrangement met with in many reptiles. The squamosal is sometimes, as in the fowl, united with the postorbital process of the frontal. In the Carinatae the quadrate articulates with the cranium by a double convex surface, in the Ratitae by a single one. The premaxillae are always comparatively large bones, the maxillae on the contrary are small, but give rise to important inwardly-projecting maxillo-palatine processes.

The relations of the palatines, pterygoids, maxillae, and vomers vary considerably, and on them Huxley has based a classification of birds[106]. In the Ratitae and the Tinamous (Tinamidae), among Carinatae the vomers unite and form a large broad bone, separating the palatines and the pterygoids from the rostrum. Huxley uses the term Dromaeognathous to describe this condition. In all other Carinatae the vomers are narrow behind, and the palatines and pterygoids converge posteriorly and articulate largely with the rostrum. Three modifications of this condition are distinguished by Huxley, and termed Schizognathous, Ægithognathous, and Desmognathous.

In the Schizognathae the vomers coalesce and form a narrow elongated bone, pointed in front, separating the maxillo-palatine processes of the premaxillae. Waders, fowls, penguins, gulls, some falcons and eagles, American vultures, some herons and many owls have the Schizognathous arrangement. In pigeons and sandgrouse there is no vomer, but the other bones have the Schizognathous arrangement.

In the Ægithognathae the arrangement is the same as in the Schizognathae, except that the vomers are truncated in front. Passeres, swifts, woodpeckers, humming birds, rollers, hoopoes have this arrangement.

In the Desmognathae (fig. 60, A) the maxillo-palatine processes approach one another in the middle line, and either unite with the vomers, or unite with one another, hiding the vomers. Thus a more or less complete bony roof is formed across the palate. The vomers in Desmognathae are small or sometimes absent. Ducks, storks, most herons, most birds of prey and owls, pelicans, cormorants, parrots, and flamingoes are Desmognathous.

The mandible, as in other Sauropsids, consists of a cartilage bone, the articular, and a series of membrane bones, the dentary, splenial, coronoid, angular, and supra-angular, developed round the unossified Meckel's cartilage. The dentaries of the two rami are nearly always fused together, but in Ichthyornis and Archaeopteryx the two rami are but loosely united. There is often a fontanelle between the dentary and the posterior bones, while the angle is sometimes, as in the fowl, drawn out into a long curved process.

The hyoid apparatus (fig. 59, C) consists of a median portion, and a pair of cornua. The median portion is composed of three pieces placed end to end, and called respectively the os entoglossum, the basi-hyal, and the uro-hyal. The os entoglossum is shown by development to be formed by the union of paired structures and is probably homologous with the hyoid arch of fishes. The basi-hyal and the long cornua, each of which is composed of two or three pieces placed end to end, are homologous with the first branchial arch of fishes, while the uro-hyal is probably homologous with the second branchial arch of fishes. In Woodpeckers the cornua are enormously long, and curve over the skull, extending as far forwards as the anterior nares.

Ribs and Sternum.

Well-developed ribs are attached to the posterior cervical vertebrae as well as to the thoracic vertebrae. The ribs generally have uncinate processes and separate capitula and tubercula, but uncinate processes are absent in Chauna Palamedea and apparently in Archaeopteryx.

The sternum (fig. 63) is greatly developed in all birds. In the embryo[107] it is seen to be derived from the union of right and left plates of cartilage, formed by the fusion of the ventral ends of the ribs. In the Ratitae and a few Carinatae, such as Stringops, it is flat, but in the great majority of birds it is keeled, though the development of the keel varies greatly. It is large in the flightless Penguins, which use their wings for swimming. Traces of an interclavicle may occur in the embryo.

Pectoral Girdle.

The pectoral girdle is also strongly developed in all Carinatae, but is much reduced in Ratitae. In some Moas the sternum has no facet for the articulation of the coracoid, and the pectoral girdle appears to have been entirely absent; it is extremely small also in Apteryx. Clavicles are generally well developed in the Carinatae, and small ones are found also in Hesperornis, and in Emeus and Cassowaries. In the other living Ratitae and in Stringops they are absent. In some Parrots, Owls and Toucans they do not meet one another ventrally. Clavicles are especially stout in some of the birds of prey. They do not generally touch the sternum, but sometimes, as in the Pelican (fig. 63, C), Adjutant and Frigate bird, they are fused with it.

In all Ratitae the scapula and coracoid lie almost in the same straight line with one another, in the Carinatae they are nearly at right angles to one another.

Anterior Limb.

In the wing of nearly all birds the ulna is thicker than the radius, but in Archaeopteryx the two bones are equal in size. In the wing of Archaeopteryx there are three long digits with distinct metacarpals. In all other birds the digits are modified, the metacarpals being commonly fused and the phalanges reduced in number. In Palamedea and some other birds the metacarpus bears a bony outgrowth, which when sheathed in horn forms a spur.

In most of the Ratitae and in the extinct Dodo (Didus) and Solitaire (Pezophaps) the wing is very small, but the usual parts are recognisable. In Hesperornis apparently only the humerus is present; in some Moas, in which the wing is imperfectly known, the presence of the humerus is indicated by traces of a glenoid cavity. In most Moas the wing is apparently completely absent. As compared with those in other Ratitae, the wings of the Ostrich and Rhea are well developed. In the Ostrich (fig. 64, B) and Rhea, as in nearly all Carinatae, the manus has three digits, but in Apteryx there is only a single digit, the second. The Penguins (fig. 64, A) too among Carinatae have only two digits, but in their case it is the pollex which is missing. In the Ostrich the third digit has two phalanges, in all other living birds it has only one phalanx.

Pelvic Girdle.

Birds have a very large pelvis and its characters are constant throughout almost the whole group. The ilium is very large, and is united along its whole length with the sacral and pseudosacral vertebrae. The ischium is broad and extends back parallel to the ilium with which in most birds it fuses posteriorly, further forward the ilio-sciatic foramen separates the two bones. In Tinamus, Hesperornis, Apteryx (fig. 65, B, 2), and Struthio, the ischia are separate from the ilia along their whole length except at the acetabulum; in Phororhacos, on the other hand, the two bones are fused along almost their whole length. The bone usually called the pubis in birds corresponds to the post-pubis of Dinosaurs and forms a long slender rod (fig. 65, 3) lying parallel to the ischium. In many birds the ischia and pubes are united at their distal ends. This is the case in the Ostrich (fig. 65, D), in which the ilia and ischia are widely separated. In many birds the pubis is drawn out in front into the pectineal process, this is specially large in Apteryx (fig. 65, B, 5), and in the embryos of many birds. It is probably homologous with the pre-pubis of Dinosaurs but in some birds is formed in part by the ilium. The acetabulum in birds is always perforate.

In Rhea (fig. 65, C, 2) and probably in Archaeopteryx a symphysis ischii occurs, and in the ostrich alone among birds there is a symphysis pubis. In Archaeopteryx all three bones of the pelvis are distinct, but they are imperfectly known. In Ichthyornis they are also distinct, in all other known birds they are fused together to a greater or less extent.

Posterior Limb.

The tibia is always well developed and has a very strong cnemial crest. The proximal tarsals are fused with its distal end, the whole forming a compound bone, the tibio-tarsus. There is frequently an oblique bar of bone crossing the anterior face of the tibio-tarsus at the distal end, just above the articular surface of the tarso-metatarsus, this is absent in Ostriches and Æpyornis. The fibula though in the embryo and in Archaeopteryx equal in length to the tibia, is in the adult of other birds always imperfect, its proximal end is often fused with the tibia, and its distal end is commonly atrophied. In the Penguins however the distal end is complete. The distal tarsals fuse with the second, third and fourth metatarsals, forming a compound bone, the tarso-metatarsus. The first metatarsal is nearly always free but occasionally as in PhaËthon it is fused with the others. No adult bird has more than four digits in the pes. In the Penguins the metatarsals are separate, and in many birds larger or smaller gaps exist between the fused metatarsals. In most birds the third metatarsal is curved so as not to lie in the same plane as the others, but in the Penguins they all three lie in the same plane. The metatarsals are clearly separated in Archaeopteryx. In Gallinaceous birds the tarso-metatarsus bears a bony outgrowth which is sheathed in horn and forms a spur.

In most birds the first four toes are present while the fifth is always absent. The first toe commonly has two phalanges, the second three, the third four, and the fourth five. In Swifts the third and fourth toes have only three phalanges. Many birds, such as all Ratitae except Apteryx, have only three toes, the hallux being absent; in the Ostrich the second toe is also gone with the exception of a small metatarsal, so that the foot retains only the third and fourth digits, the third being much the larger of the two and bearing a claw, while the fourth is clawless.

In the Swifts, Cormorants, and Penguins, all four toes are directed forwards. In most birds the hallux is directed backwards, and the other toes forwards. In the Owls the fourth toe can be directed backwards as well as the hallux, while in Parrots, Cuckoos, Woodpeckers, and Toucans the fourth toe is permanently reversed. In Trogons the second toe is reversed in addition to the hallux, but not the fourth.