THERE are, broadly speaking, two sets of characters to be found in the skeleton of any animal. There are, first, those which bear a direct relation to its position in the class of vertebrates, bear witness to its origin and relationship, and are shared to a greater or less extent by every member of the group to which it belongs. These are termed structural or morphological characters.
There is also another set of characters which are connected with the animal’s habits or mode of living and are believed to have been acquired during the development of the species, and are called secondary or teleological. It is owing to the fact that the skeleton is influenced by these two great factors that it is possible to tell from the skeleton, or even from parts of it, not only what position the animal holds in the scale of life, but what were its habits as well. So the modifications of a bird’s skeleton are primarily morphological, those due to the fact that it is a bird, but these are associated with others rendered necessary by the adaptation of the fore limb for flight and its consequent withdrawal from use in any other form of locomotion, or for taking food. And in birds these last so overlay the others that the classification of birds is a very difficult matter. This is well shown by the long use of the divisions Ratite and Carinatae, which depended mainly on the presence or absence of a keel to the sternum, when this and a number of other characters depended upon whether a bird did or did not fly. So the structure of a bird’s skeleton depends first upon the fact that it is a bird, secondarily upon the manner in which it moves about, and thirdly, to a still less degree, upon the way it gets its living. Flight in itself is not a distinctive character, for mammals fly today, and reptiles, in the shape of pterodactyls, flew ages ago, having successfully mastered the problem of flight about the time the bird had taken its first lessons in the art. But the manner in which the fore limb is modified, the ground plan, so to speak, on which it is built, is a primary morphological character and differs in the birds, mammals, and reptiles.
In the Bat the four fingers of the hand are greatly lengthened for the support of the membrane forming the wing; in the flying reptile the membrane is supported wholly by the enormously developed fifth or little finger. In the bird the bones of the hand are lessened in number and peculiarly modified for Wing bones of a Pigeon. the attachment of feathers which form the highest type of wing.
The general characteristics of a bird’s skeleton are lightness, length of neck, very decided difference between the fore and hind limbs, and reduction in the apparent number of bones of the hand and foot (metacarpals and metatarsals) by their fusion with one another. The skull joins the neck by a single condyle as in reptiles, and, also as in reptiles, the ankle joint is between the bones of the ankle and not between the leg and ankle as in mammals. The jaw is not attached to the cranium directly as in mammals, but by a free quadrate as in snakes and some extinct reptiles. In existing reptiles other than snakes the quadrate is fixed. Many ribs of the chest cavity bear little processes directed upwards and backwards, termed uncinate processes, and these are found in all birds save the Screamers, although almost wanting in the Secretary bird. Outside the class of birds such processes occur only in that curious New Zealand reptile, the Hatteria, and, in cartilage only, in Crocodiles. So, in many important particulars the skeleton of a bird resembles that of a reptile, and this led Huxley to unite the two in a common superclass, Sauropsida.
This possession of characters in common with reptiles is why it is considered that if birds have not been directly derived from reptiles, they have had a common origin. From the strong resemblances between birds and dinosaurs it was long thought that these reptiles were the parent stock of birds, but this theory has been practically abandoned.
Professor Seeley has thought that the resemblances between birds and pterodactyls was more than superficial, but he stands practically alone in this view, the most commonly accepted working theory being that birds and dinosaurs have had a common ancestry.
The extent to which the skeleton of a bird is permeated by air usually bears a direct and apparent ratio to its mode of life. Thus the Condor and other soaring birds, such as the Frigate-birds, Cranes, and Screamers, have very lightly built skeletons, and Ducks and other water fowls have the cavities of the long bones filled with marrow, while the bones of the strictly aquatic Penguinsare filled with bony tissue.
That the lightness of the skeleton Wing bones of a young Ostrich. (After does not necessarily appear in connection with the power of flight is shown by the Hornbills and especially by the larger species, for in these birds of heavy, lumbering flight, the air penetrates to the very tips of the toes. On the other hand, in birds like the Condor and Frigate-bird this pneumaticity, or presence of air in the bones, is believed to aid in oxygenizing the blood and in adjusting the air pressure when a bird descends rapidly from a great height. It may also be connected with lessening the sudden shock that takes place when a Gannet or Brown Pelican plunges headlong into the sea.
It is usual to commence the description of a skeleton with the skull, but while the skull is of the utmost importance to the systematist, it is a complicated structure whose topography is by no means easy to understand, and whose numerous parts bear equally numerous and unfamiliar technical names. So we may slight this, leaving it to be briefly described later on, and begin with the wings, which, next to the feathers, are the most obvious features of a bird.
The wings of a bird comprise the same parts as the fore limb of a mammal or reptile, and save in the hand, we can readily recognize these various parts, the upper arm (humerus), forearm (radius and ulna), wrist, and hand.
The bones of the fore limb are modified for the support of the large feathers forming the wing, the hand being reduced to three fingers, while only two of these, those to which the primaries are fastened, are of much use.
The wrist of the adult bird, frequently called shoulder, consists of but two bones; the part corresponding to the palm of the hand comprises three metacarpals and two carpals solidly fused into one mass, though this can be seen only in a very young bird. Following this is a short finger on the front edge of the wing bearing the so-called bastard wing, one long central bone and one shorter more or less pointed. The stages by which the clawed hand has been transformed to a wing may be gathered partly from fossils and partly from a study of the embryo, but it is not quite certain whether the first finger of a bird, that bearing the so-called spurious wing, corresponds to the first or second finger of man,with the probability in favor of its being the second. The wing bones are lengthened or shortened in a pretty direct ratio to the rapidity with which the wings are moved, being longest in such sailing birds as the Albatross and shortest in the Pigeons and Hummingbirds. The shortening is greatest in the humerus; for while in the Albatross and Frigate-bird the upper arm and forearm are about equal, in the Pigeons the humerus is somewhat shorter, and in the Hummingbird very much shorter, than the succeeding bones. This relates to the fact that a bird’s wing is a lever of the third with the the fulcrum, the muscle the power, the end of the wing the weight. The shorter the wing, the easier it is to move it rapidly; the more rapidly it is moved, the stronger must it be. The wing of a Condor or Albatross would break, were sufficient power applied to move it as fast as that of the Pigeon. The rapidity of the wing stroke is also indicated by the development of the processes about the inner end of the humerus for the attachment of wing muscles, these reaching by far their greatest development in Hummingbirds.
The wing is supported by the coracoid, a bone abutting on the front of the breast-bone and raking forwards and upwards. This bone, represented in the higher mammals by a mere process on the shoulder, forms half of the shoulder joint in reptiles and also in the Echidna and Platypus, and by far the greater part of the shoulder joint in birds, that part of the shoulder blade being much reduced.
In perching birds the coracoid is long and slender, and in birds which soar or sail it is shortened and broadened, for it is a rather curious fact that while the amount of muscular power employed in flight is much smaller in sailing birds than in others, the support for the wing is much more strongly built. In the Frigate-bird, which is perhaps the most expert bird of flight, the area of wing muscle is proportionately smaller than in any other bird, while the wishbone is united at its apex with the breast-bone and soldered to the coracoids at the other end, thus forming a rigid support for the wing. The shoulder blade is slender and as a rule more or less pointed, the Penguins being exceptional in having a broadly expanded scapula, while the Woodpeckers are characterized by having the end bent downwards. Attached to the coracoids in front is the wishbone, which represents the clavicles or collar bones of other animals. This is usually “U”-or ” V “-shaped, with the apex near the keel of the sternum or even united with it, as in the Steganopodes. In some Parrots and Toucans the upper portions only of the clavicles remain attached to the coracoids, and in the struthious birds, save Emeu, clavicles are entirely lacking. In birds of prey the clavicles are broadly “U “-shaped and heavily built, serving to brace the wings apart, but in the majority of birds they are of little structural importance, and in such admirable flyers as the Hummers are practically of no use.
The breast-bone, sternum, bears a direct relation to habits, and to a less extent is valuable in classification. While the terms Ratitae and Carinatae, keelless and keeled, are convenient in forming a key, and the corresponding conditions were formerly held of primary importance in classifying birds, they have been abandoned by the best anatomists, as they do not express the truth. The development of the keel of the sternum bears a direct relation to the extent to which a bird moves its wings, whether in flight or swimming. Birds which do not fly have the keel of the sternum small or absent, according as the power of flight has been lost, geologically speaking, for a longer or shorter period of time.
The members of the Auk family fly somewhat heavily, bone. owing to the small size of their wings, but as these are used for flying under water as well as above it, the breast muscles and sternum are large. For the same reason the Penguins, which do not fly at all, have a large sternum, since they swim entirely with their wings, presenting in this respect an analogy to the eared seals, which swim with their fore limbs.
The keel of the sternum is very much reduced and the body of the sternum greatly shortened in birds which sail, this mode of flight involving the expenditure of comparatively little muscular energy. The Albatross has a small breast-bone, and the Frigate-bird smaller still, and these birds are those which fly with the fewest movements of the wing. On the other hand, birds that fly by strokes of the wings have large breast muscles and a correspondingly large sternum, these reaching their maximum in the Hummingbirds, whose skeleton when brought up to the size of a Pigeon is seen to be very powerfully built. The Pigeon, by the way, exhibits the development of the sternal keel for powerful flight.
The front part of the sternum bears the coracoids; the ribs are attached to its sides, while the body of it supports the viscera. In all water birds the breast-bone,of Birds is long, corresponding to the long bodies of these birds. The hinder portion of the breast-bone may be entire, perforated, or notched, the notches being two or four in number, reaching their extreme in the fowls in which the body of the sternum is very small and the lateral processes extremely long and slender.
The front of the breast-bone may bear a projecting process, or “manubrium,” and this may be developed from the inner face of the bone, spina interna, or outer, spina externa, at the region of the keel. The manubrium may be a spine (Curassows), or low projection (some Owls), while the extreme development is found in the long “Y”-shaped process so characteristic of the Passeres, the Woodpeckers coming next in this respect. These characters are apparently not associated with any corresponding modifications in the habits of birds, and are, therefore, of great importance in determining the affinities of various groups.
On either side of the breast-bone are little prominences to which the ribs articulate, and in many birds these articulations are found well forward and on a triangular-shaped process, termed the costal process.
The uppermost bone of the leg, the femur, is always short, even in wading birds, and usually pneumatic, or permeated by air. The extreme of shortness and width is found in the extinct diving bird Hesperornis, in which the femur suggests that of a seal.
The knee-pan, or patella, is usually small, except in such swimming birds as Cormorants and the extinct Hesperornis, where the head of the tibia is short, and it is largely developed to serve for the attachment of muscles. In Grebes and Loons the upper end of the tibia is greatly extended and the knee-pan correspondingly reduced, appearing as a small splint of bone back of the process.
The tibia is much the larger bone of the lower leg, the fibula being flat and splint-like in character, never quite reaching the lower end of the tibia and commonly not extending more than two thirds of its length. The length of the tibia is related to a bird’s habits, being longest in wading birds, coming next in runners, and of considerable development in swimming birds.
The foot of a bird never contains more than four toes, and there may be but two, as in the Ostrich, while the three principal metatarsal bones are united into one, with which the second row of tarsal bones is fused, this forming the tarsometatarsus, or, as it is commonly called, tarsus. The bones corresponding to our heel bone, calcaneum, and its neighbor, astragalus, unite with the tibia, so that the ankle joint of a bird, like that of reptiles, is between the bones of the ankle, and not as in mammals between the leg and ankle.
The upper end of the tarsus and its relation to the tendons is a fair index to the position of its owner, being simplest in Ostriches and other birds undeniably low or generalized in character, and more complicated in higher forms, reaching its greatest complexity in the perchers, in which the hypotarsus, as it is termed, is pierced for the passage of four or five tendons.
In the Crow, for example, there are four large and one small perforation, in the Clamatores but four, and one of these is closed by cartilage and not by bone; in the Picariae there are but one or two tendinal perforations, and in the divers but one, so that the specialization of the tendons and that of the tarsus go together. The three divisions of the lower end of the tarsus indicate that it is composed of three bones, but these bones are clearly shown only in embryos, in young Ostriches and in Penguins, where the bones, though united, are plainly indicated throughout life, this retaining of a primitive condition being one of the characters which has led many good authorities to place the Penguins in a group contrasting with all other fan-tailed birds.
The leg attaches to the hip-bone, or pelvis, each half of which is composed of the usual three bones, ilium, ischium, and pubis, although these fuse together at an early date and show as separate bones only in very young birds.
In all birds the pubis is directed backwards, and the greater part of the ilium lies in front of the hip-joint, this being a point where existing birds differ from existing reptiles. In the lower birds, as the Ostriches and Tinamous, the two principal bones of the pelvis, the ilium and ischium, are free from one another behind, this being a primitive character in which these birds resemble reptiles. The Cassowaries are an exception in having these bones united.
In the vast majority of living birds (the Neognathae) the ilium and ischium are firmly united. The pubic bones, the long, slender, lowermost bones of the pelvis, unite posteriorly in the Ostrich, but are free in other birds and are frequently widely separated; they may even be nearly lacking, as in Eagles, in which the hinder portions only remain. The outward flare and generally open character of the pelvis below has to do with the question of room for the passage of the large and brittle egg. Birds that fly much, and especially sailing and soaring birds like the Petrels and Frigate-birds, have a broad, short pelvis,while in water birds it is long and narrow, much the same condition being found in flightless birds which run much.
The neck vertebrae of birds are peculiar from the character of their articulations, which are saddle-shaped, concave one way and convex the other, a form termed heterocoelous, and one that allows great freedom of movement in both planes. Theoretically the ball-and-socket joint permits the greatest amount of motion, but in practice this form of joint is usually combined with some arrangement which checks its movements. Thus in the snake, while there is the utmost freedom of movement from side to side, there is but little play vertically. The neck of the bird is always long and the vertebrae numerous; this is necessary in order that the bird may reach all parts of its body with the tip of its bill, and secondly, that it may obtain food. Thus long legs and long necks go together, or, as in the Sandpipers and Snipe, there is an increase in the length of the bill, while Swifts, Swallows, and Goatsuckers, which capture their prey in mid-air, have short necks. Swans, which do not dive, have much longer necks than diving birds, or Geese, which feed largely on land. The very long neck of the Darter is associated with its habit of suddenly straightening the neck and impaling fish on its sharp bill.
Usually the last two neck vertebrae bear free ribs, these being but the lengthening and freeing of the long processes running backwards from the sides of the front of the vertebrae. That these are really ribs may be readily seen in a young Ostrich, in which they are free, but while later on they become united with the bodies of the vertebre, the last two remain free, although they do not reach the sternum.
The thoracic region of a bird, the body proper, usually consists of a comparatively small number of vertebrae bearing long ribs, the foremost of which are attached to the sides of the breast-bone. Water fowl, like the Loons, Auks, and to a lesser degree Ducks, have the longest bodies, soaring birds the shortest. Several of the vertebrae in the center of the series are fused or ankylosed together to stiffen the body for flight, a free vertebra or two next- the pelvis permitting some motion here.
The sacral vertebrae, or those to which the pelvis is attached, are really but two, as in reptiles, or in rare cases three in number, but in front and behind are added vertebrae from the back and tail region, the result being a long series of vertebrae firmly united in one mass and furnishing ample attachment for the pelvis and a firm support for the legs. The number of bones in this synsacrum, as it is called, can only be clearly seen in young birds, but there may be from twelve to as many as twenty.
Finally comes the caudal series of free tail vertebrae, which may vary from five to as many as ten, the average number being about six, and in all the higher birds, or Neognathae, terminating in the flattened urostyle, to which the tail feathers are attached. This bone in turn comprises four to six modified sections.
The skull of a bird, roughly speaking, is divided into two parts, the beak and brain case. The beak portion, which is very directly concerned in the getting of food, is subject to many and great modifications; the rear portion, being away from direct modifying influences, is less altered, and here the palatal part of the skull is of the most importance for purposes of classification, for the less a part is subject to outside influences and the less a creature’s habits have to do with any part of the body, the less, theoretically at least, should that part be subject to modification, and the more important it is for use in classification.
There are two existing types of skull structure by which birds are divided into two great groups, the Paleognathae or Dromeognathae, including the Ostriches and their allies, and the Neognathae, Euornithes, or Eurhipidurae, which comprise the vast majority of birds. These types are sharply marked, indicating that at the very outset of their career, birds split into these groups, if indeed they may not have originated from two distinct types of reptiles. Unfortunately, as previously noted, these modifications can be described only in technical language and can probably be best understood from a study of the accompanying figures.
In the dromaeognathous type of skull,so called because it is typically found in Ostriches, the vomer is broad and unites in front with the maxillopalatines, while behind it receives the posterior extremities of the palatines and the anterior ends of the pterygoids, which are thus shut out from joining the sphenoid; the sphenoid bears on its sides long basipterygoid processes which give it something of a cruciform shape. In birds with this type of skull the quadrate, the bone to which the lower jaw is joined, is rather short and clumsy and its articular head is single or but faintly divided into two portions; the quadrate is also locked into place by the surrounding bones. In all these particulars the dromaeognathine skull more nearly resembles that of a reptile than does that of the majority of birds, a point that may be best appreciated by comparing the figures.
In the euornithic type of skull the palatines articulate with the pterygoids and both touch the sphenoid at their point of junction, and the back of the vomer embraces the sphenoid between and above the ends of the palatines. The quadrate has two heads and is loosely joined to the cranium. This arrangement prevails in the majority of birds, and is termed the euornithic type of skull because it is characteristic of the Euornithes; it is also called neognathic because it is believed to be more recent or newer than the Ostrich style and further removed from the reptilian skull.
Basipterygoid processes may be present, but usually in the form of low facets which articulate with projections on the pterygoids and often serve as braces to the beak when this is slightly movable, as in Ducks and Parrots.
The neognathous style of skull is subject to several important modifications which characterize great natural groups of birds. These are the schizognathous, desmognathous, and aegithognathous types,’ which may be briefly characterized as follows : When the vomer is pointed in front and entirely free from the maxillopalatines, and these are free from each other, the skull is termed schizognathous; when the maxillopalatines are expanded and fused with each other, the vomer being small or absent, the skull is desmognathous; when the vomer is expanded in front and free from the maxillopalatines, and these are slender at their point of origin and disjoined, the skull is said to be aegithognathous.
The second of these types, the desmognathous, is to some extent a .modification of the first, brought about by the development of bone in the palatal region which binds the various parts together and hides its real structure. Thus desmognathism occurs in varying degrees in birds obviously closely related, while the transformation of one type of skull into another is admirably illustrated by the Cormorant. This bird has at first a schizognathous skull, but by the time it takes to the water, growth of bone in the palatal region has converted it into a completely desmognathous cranium.
Two other features remain to be considered in connection with the skull, the hyoid, and those modifications of the nasal bones and narial openings termed by Garrod holorhinal and schizorhinal. In the holorhinal type the openings are more or less oval, the posterior border curved and lying in advance of the posterior ends of the premaxillaries. In the schizorhinal type the openings are more or less elongate, with the posterior border angular or slit-like and lying back of the posterior ends of the premaxillaries.
These features are, to some extent, valuable in classification, but by no means of the importance at first ascribed to them by Garrod, being one of the many emphatic warnings that birds may not be classified by any one set of characters, but by the resultant of many.
The beak part of the skull, as just stated, is subject to great modifications connected with the taking or manipulation of food, and may be long and slender like a probe, broad and flat, or short and strongly made for crushing seeds. The extent to which habit and modification may go is shown in the Pelican, Gannet, and Cormorant, and in all these diving birds the nostrils are completely filled up and the roof of the mouth strengthened by the growth of the bone.
The Cormorant starts in life with open nostrils, but by the time it is ready to take to the water and seek food for itself, the nostrils are completely closed by the growth of bone and overlying horny beak. These changes are accompanied by others in the bones of the palate by which the structures of this region are entirely changed and its real characters obscured.
The hyoid, all but the front portion, corresponds with the first gill arch of a fish and supports and controls the motion of the tongue. The tongue itself is built upon the front portion of the hyoid, which is subject to little modification, but the extent to which the tongue can be protruded depends on the length of the bones of the hinder part. Hence in the Woodpeckers, which use their tongues as probes, the hinder parts of the hyoid curl up around the back of the head and may even, as in Colaptes, pass over and into the nasal chamber nearly to the tip of the beak. Thus from bill to toes, not merely the external form of the bird, but the underlying skeleton as well, is fashioned to adapt the bird to its surroundings. As William Kitchen Parker used to say, “Adaptation, adaptation is the keynote to the structure of a bird.”