Part 21

B, Barbs; bp, proximal barbules; 1, flange; 2, ‘dog-tooth,’ part of flange; 3, overlapping portion.]

B, barbs; bd, bp, barbules (distal and proximal).]

The most curious modification, however, of the feather is into that structure known as a ‘powder-down.’ These feathers have, as their name denotes, a powdery appearance, which is due to the continual breaking off of the fine ends of the barbs; the feathers themselves are soft, and belong to the variety of feathers which have been described as down feathers. The dusty matter which they give off has been described as ‘dry and yet fatty to the touch.’ They are found in various birds; they do not characterise any one particular group, except the Heron tribe; some Parrots have them, a few Hawks, and certain other genera. It has been said that they are phosphorescent; and it has been suggested that their presence in the heron is of use to it in its fishing. The light, it is thought, attracts the small fishes within reach of the heron’s long bill. But this appears to be one of those exaggerations founded upon actual fact which are so common in natural history.

Another important fact about a feather is its colour. There is no purely white bird in this country and not very many that are chiefly white. But there are some, like the Gulls and the Storks. The nearest approach to an absolutely white bird is the beautiful little Egret, whose plumes are, unfortunately, so much used in feminine adornment. As concerns its feathers, this bird is absolutely white, but other parts of the body are black. A bird that is purely white, not only in the feathers but in the legs and beak, is called an albino. This state of affairs is not commonly met with, but it sometimes occurs; everybody has heard of that contradiction in terms, but actually existent creature, the ‘white blackbird.’ In all these cases there is something wanting in the feather; for white is not a colour--it is the negation of colour, and is due in nearly every case to the scattering of the rays of light which fall upon the object. This happens when the material that is coloured white is broken up into minute fragments separated by air. The froth of the sea or of a brimming tankard is simply due to the entangling of bubbles of air, which scatter the rays of light. The stems of the feathers contain bubbles of air, which bring about a like effect. But the majority of birds are coloured, and, as a rule, perhaps, brightly coloured. We have not in this country many birds which can compare with the gaudy parrots of the East; but brilliancy of hue is by no means wanting in the birds of this and of other countries which enjoy a temperate climate. It used to be said that brilliancy of colour was a characteristic of the tropics. But it is always pointed out, by way of a refutation of that statement, that the Golden Pheasant of China is as gorgeous a bird as any which exists. There are few small birds which are really more brilliant in hue than our Yellow-hammers, Goldfinches, Bullfinches, and some others. We have, it is true, nothing to seriously compete with the Humming-birds; but these birds are found not only in the tropical forests of Brazil, but also in North America and upon the snowy summits of the Andes, and can therefore hardly be used as an instance of the exclusive restriction of brilliant colour to a tropical climate.

The hues of the feathers are due to two causes. In every case where there is colour at all the feathers contain a certain amount of dye, or pigment, as it is more usually termed; this pigment may be alone responsible for the colour of the feather, or it may be only a part of the cause. If the bright blue feather from a Macaw’s wing be roughly pressed so as to injure the surface, the blue colour will disappear from the rubbed place, and will be apparently replaced by a brownish black. The reason for this is that the blue colour is the result of the actual structure of the feather, which requires the underlying black pigment for its manifestation. The crushing destroys that structure and leaves only the dark pigment. The brilliant and varying hues of the soap-bubble and of mother-of-pearl are examples of substances which owe their colour to their structure; and the hues of the bird’s feather are produced by a similar kind of structure. Finely ruled lines engraved upon the feather just below a clear and transparent outer skin are responsible for the tints of different colours. But there are many birds whose colours are entirely due to the pigments. The most interesting instance of this in many ways is an African bird, the Touraco. This bird is green for the most part, but the feathers of the wings are of a magnificent crimson. When the birds take to the wing this gorgeous colour is displayed; before, it is concealed by the overlying feathers. The colouring matter can be easily extracted from the wing, and it forms a solution of a splendid crimson as bright as the substance called cochineal, which is the product of an insect. It was once said that this colour could be, and was as a matter of fact, washed out from the wings of the bird during heavy storms of rain, and that when a touraco was shot and fell into the water it stained the water red, not with its blood, but with the dye from its feathers. This is, however, an exaggerated way of putting the fact that even very feebly alkaline water will dissolve out the colour. Some of the yellows of the woodpeckers and the browns and reds of other birds are solely brought about by the presence of pigments.

In speaking of birds as ‘feathered songsters’ or as ‘feathered bipeds,’ we are a little apt to lose sight of the fact that they are also scaly--an error which is occasionally rectified by the view of an obtrusive pair of legs belonging to the fowl upon the dinner-table. The legs of birds are nearly always scaly; there are a few exceptions or nearly exceptions. For instance, there is a special breed of pigeons with feathered legs; and the sand-grouse, which makes those remarkable and periodical invasions, has legs which are more covered with feathers than with scales.

The possession of scales is one of the most striking points of resemblance between birds and reptiles. At first sight it seems to be almost absurd to attempt to draw any parallel between the active, feathered, hot-blooded bird and the scaly, cold-blooded reptile; yet there are many resemblances, some others of which will be indicated in the following pages. In the meantime we are concerned with the scales. These are flat plates, produced by a horny alteration of the soft underlying skin, which are precisely like those of the lizards and snakes. No other animals possess scales; those of the armadillo appear to be not unlike the scales of reptiles and birds, but they really are not, nor are those of the scaly manis, which are more comparable to closely matted tufts of hair. The scales of a fish are totally different, since they are not formed by the true skin, the epidermis, at all, but by the underlying dermis. In no bird, however, are there scales upon any part of the body except the legs. But one bird makes a near approach to having scales elsewhere. This is the Penguin, the feathers of whose wings are flattened and very scale-like. But the characteristic fringing of the feather can be detected on a careful examination. The penguin uses its wings as paddles to fly under water. A branching and delicate feather would be worse than useless under such circumstances; hence the superfluous fringing of the stem of the feather has been got rid of, and the feather itself has become flattened and lies close to the skin.

Beak.

The beak is simply a horny tract of skin which has become hardened for its special uses. It is not even distinctive of the bird; for turtles, particularly the snapping turtles, have beaks which are not only precisely like those of birds, but are equally effectual when turned to aggressive ends. It is a commonplace of knowledge that the bill or beak presents an almost endless variety of form, which is associated with an equally diversified use. The remarkable shovel-shaped bill of the duck is suitable for dabbling in soft mud, just as is the hooked beak of the hawk or owl for tearing living prey. The most prevalent form of bill is that possessed by most passerine birds, a conical longer or shorter bill. The relatively enormous beak of the toucan is serrated along the free edge, which enables its possessor to obtain a firmer grasp of the fruits upon which it feeds. The ridges upon the inner surface of the beak in the ducks serve an analogous purpose; the same structure is seen in the bill of the Flamingo, though the outline of the bill is unlike that of the duck, and gave rise to the idea, or at any rate had something to do with the former impression, that the flamingo was a long-legged duck. But, as a matter of fact, there is a stork in which there is precisely the same ridging of the beak, and it is more usual now to place the flamingo among the storks, or near to them. The Spoonbill, as its name denotes, has a beak which is at the extreme of the series of beaks which are useful for sifting the mud at the bottom of pools and rivers; the extremity is widened and flattened out. Most singular is the recurved bill of the Avocet, and equally so the under-jawed Rhynchops, the terms used implying the peculiarities in each case. There is no living bird which lacks a beak; but in some of the extinct and toothed birds, which are again referred to later, the beak was absent. Its place was taken in them by the teeth.

Feet.

Hardly less diversified in form are the feet of birds. The skeleton of this part of the body is dealt with on another page; here we are concerned only with the external form of the feet and legs. Aquatic birds often have webbed feet, but not always. The Dipper, for example, is a bird which lives largely on and under the water, but its feet are not in the least like those of a Duck or Grebe. The webbed foot presents us with at least two varieties. In the Pelican tribe (fig. 5) the extreme of web-footedness is to be seen. Here all the toes (four) are connected by a webbing. In the Duck only three of the toes are webbed. Another kind of webbed foot is termed palmate. In the Coots, for example, each toe is fringed with a broad membrane, but there is no connection between the fringes of successive toes. The toes of birds are apt to be differently disposed. In most birds (fig. 6) there are three toes which are turned forwards, and one, the great toe (hallux), which is turned backwards. But in the Trogons and others two toes are turned forwards and two backwards, thus producing a very efficient mechanism for holding on tightly to the bough of a tree, a mechanism which is shared by that, in some other respects, bird-like lizard, the chameleon. A foot of this kind is technically called ‘zygodactyle.’ A singular modification of the foot is seen in the Kingfisher (fig. 7) where the two middle toes are enclosed in the same fold of skin; this is called ‘syngenesious.’

Skeleton.

A bird’s skeleton is wonderfully light and spongy in texture. It is full of air (see below, p. 27), but deficient in marrow. Its entire structure is pre-eminently suited to a flying creature, not only for the above reasons, but because the heaviest part (the sternum) lies in the middle, in the centre of gravity, and thus assists in preserving the balance, like Blondin’s pole.

The Skull.

The skull of a bird is composed of a large number of separate bones, which are very closely united in the adult bird, so much so that it is next to impossible to recognise that they are distinct bones. The bones are also thin and light, for to a flying animal any weight forward would be most disadvantageous. The weight of the bird should be, and is, concentrated in the middle of the body. We can divide the skull into two regions: behind is the smooth, rounded brain-case or cranium; in front is the face, which is largely ensheathed by the beak. It is chiefly formed by the maxillary and nasal bones above, and by the palatine and pterygoids below. The length of this part of the skull is subject to great variation in different birds. In the Storks, for instance, the face is extremely long, while in the Parrots it is comparatively short.

Professor Huxley, about thirty years ago, proposed to classify birds by the form of the bones of the palate. In the skull of the Hawk, it will be seen that two bones lying in the front region of the palate are fused with each other in the middle line, and to the type of skull which is thus characterised the name ‘desmognathous’ was given. It is found not only in the Hawks, but in a quantity of other birds; for instance, in the Stork tribe, and in the Hornbills and Toucans. The second form of skull distinguishes the gallinaceous birds; in them the two maxillo-palatines remain unconnected, and the palate is therefore in a way cleft; this is termed the ‘schizognathous’ skull. In the finch tribe there is a slight modification of this, called, from the Greek word for a finch, ‘ægithognathous.’ In these birds a median bone, called the vomer, from the fact that the bone to which it corresponds in the human skull is shaped somewhat like a plough-share, is truncated in front, instead of tapering, as it does in the schizognathous skull of the common fowl. There is a fourth variety, which marks out the Ostrich tribe and the American Tinamous, in which the two pairs of bones called the pterygoids and palatines do not, as they do in the types of skull that have been hitherto considered, reach the middle line of the skull, but are kept off from it by the vomers, which extend backwards. The term ‘dromæognathous,’ or emu-like, is applied to this form of skull. If the back of any bird’s skull be examined, it will be noticed that just below the great hole or foramen, through which the medulla passes to join the spinal cord in the canal of the vertebral column, is a rounded, rather kidney-shaped boss. This is the occipital condyle, by means of which the skull articulates with the first vertebra. If you look at the same region in a mammal, you will find that there are two of these, one on each side, though also below the foramen magnum. This is one of the many points of structure that distinguish a bird from a mammal and ally it to the reptiles; but it must be remembered that in some reptiles there is a commencing division of the single condyle into two.

The Vertebral Column.

Like all other backboned animals, birds have a chain of small bones running along the back, and enclosing a canal in which runs the spinal marrow. In most vertebrates some of the individual vertebræ in the region of the hind limb, the sacral region, are somewhat intimately fused together, forming a more solid structure for the support of the pelvis. In birds the strong coupling of the vertebræ is more marked, and extends to the dorsal region. The mechanical value of this to a flying animal is clear; it is analogous to the tight coupling of an express train, and prevents the back from bending from side to side under the strain produced by the powerful movements of the muscles in flight. The tail vertebræ show some curious modifications in different birds. In the typical carinate bird, the last few vertebræ are fused into a piece which is called the ‘plough-share bone,’ or ‘pygostyle.’ The name of this bone sufficiently indicates its shape; the expanded end of the bone serves as a firm base, upon which rest the strong tail feathers. Now, in the ostrich tribe there are no rectrices comparable in size to those of the flying carinates. Here there is no pygostyle, but the individual vertebræ are small and disconnected. They are, however, few in number, whereas in the Archæopteryx they are numerous, though, oddly enough, not so numerous altogether as are the tail vertebræ of some flying birds. Each individual vertebra in the Archæopteryx supports a pair of rectrices, which are thus arranged in a series, and not in one row. A very distinctive peculiarity of the vertebræ of birds is the saddle-shaped centrum. The centrum of the vertebra is the solid piece which underlies the canal of the spinal cord, the walls of the latter being formed by the neural arches, which unite above to form a neural spine. In other vertebrates the centra are flat (mammals), or procœlous (the concavity being forward), or opisthocœlous (the concavity posterior), or amphicœlous (concave on both sides). This latter form of vertebra is frequently met with in archaic forms belonging to various groups. It occurs, for example, in many fishes. Such reptiles as Hyperodapedon and the Geckos have the same kind of vertebræ. Among birds there is no existing genus or species which is to be thus characterised; but the extinct Ichthyornis had clearly biconcave vertebræ.

Shoulder Girdle.

h, ribs; 58, furcula; 52, coracoid; 59, anterior end of sternum.]

This series of bones serves as the intermediary between the fore limb and the vertebral column. It consists of three distinct elements. There is, first of all, a sword-blade-like bone with sharp edges, which lies along the vertebral column--the scapula. To the end of this is firmly attached a somewhat shorter bone, which approaches its fellow as it joins the sternum below; this bone is known as the coracoid (52, fig. 8). The angle between these two bones is, in flying birds, a considerable one, but in the ostrich tribe they are almost in the same straight line; this is really connected with the power of flight, for it has been shown by careful measurements that, in birds which still have wings that bear every appearance of being functional, and yet are not used for their legitimate purpose, the angle tends to approach the obtusity of the scapula and coracoid of the Ostrich. Birds have, besides these two bones, the merry-thought, or clavicle (58, fig. 8), which corresponds to our collar-bone. Its two halves are generally closely united to form one [**Symbol:]U-shaped or [**Symbol:]V-shaped bone; but sometimes they are separate, and then more or less rudimentary.

Wing.

The second digit (II) is free, being prolonged beyond ala membrane (P.m.), and remiges 8–10 are not developed.]

The hand is shorter, and not fitted to be a grasping organ.]

The hand is smaller relatively to the forearm; c, the claw of digit I, much reduced.]

I, II, III, digits; R., U., D.c.f., carpal bones; Mc., metacarpals.]

We must enter into the matter of wing a little more closely--it is so important a feature of bird organisation. The wing, of course, although it performs so different a rôle, is the exact equivalent of the fore limb of mammals. We can easily recognise precisely the same bones, though they are diminished in number, and often of a different form. It will be noticed that in each case we can distinguish the three bones forming the arm, and which are known as the humerus, the radius, and ulna. The rest of the limb in the bird is not quite so obviously like the hand of the mammal; but a little attention will show that it is constructed upon a perfectly similar plan. The flexible wrist of the mammal is made up of many small bones; the hand itself is made up of a larger series still, of which those nearest to the wrist are technically termed the metacarpals, and those which follow, the phalanges. In many mammals there are five fingers; but there are many which have less, and the extreme is reached in the horse, which has to put up with a single finger and small rudiments of two others. Now the bird is better off in the way of fingers than the horse, as it has three fairly well-developed fingers, or rather two well developed and one less perfect. The shortest finger corresponds to the thumb of our hand. It is more freely movable than the others. The metacarpal bones of the second and third fingers are firmly welded together, and are long; each finger (as will be seen from a look at fig. 1, p. 4) has one or two phalanges, as the case may be. Now in mammals the end phalanx of each finger is tipped with a nail, or with a hoof. The powerful claws of the tiger, used for tearing, and the solid hoof of the ox or horse, upon which the creature walks, are one and the same thing. It might be supposed that the hand of the bird, which is not an organ of offence or meant to walk with, might be shorn of these appendages. But this is not the case: every bird has at least two nails (fig. 9), of a long and rather claw-like form when well developed, and sometimes three nails, that is, one to each of its fingers. It looks, therefore, very much as if the wing of the bird had been formed out of a limb that was once an organ for climbing or walking with. There is a curious bird, found in British Guiana, which is known as the Hoatzin (figs. 9, 11). In the very young nestlings of the hoatzin the claws of the fingers are so conspicuous that they are actually used by the callow chick to climb with, before the feathers of the wings have grown sufficiently to enable them to use their wings in the proper way in which a bird should; it has been said also, that other birds scramble about and use their claws when they are young. In the case of the hoatzin, it is stated that the thumb and the first finger can be brought together so as to lay hold definitely of an object. A very important thing to notice about the wing bones is that they are capable of but little movement upon each other. There are two hinges, one at the elbow, and the other at the wrist; but the radius and ulna cannot move round each other, as they can in our arms, and the fingers are fixed and rigid. This would be most unfortunate if the wing had to be used as a walking or climbing limb; but it is most useful in relation to the function which the wing has to perform--that of flight. The strength of the downward stroke would be enfeebled if the bones were in a limp condition and moved upon each other. They offer, too, a firm foothold for the thick quills of the big feathers of the wing. It has been mentioned that all the evidence at our disposal points to the view that the wing has become gradually moulded into an organ of flight, from a condition in which it played a different part. The earliest bird of which we have any record had wings which were much less perfect as flying organs than those of modern birds. It seems pretty plain that the bones in that antique bird were much less rigidly fixed together, and it is equally clear that the fingers were very much more loosely attached to one another. They were also more on an equality as regards size; the great disparity evident in fig. 12 is not to be seen in the Archæopteryx. All this, of course, shows that the Archæopteryx could not have possessed the ample pinion of its more vigorous descendants of to-day. The fossil Archæopteryx looks a little like a crow would look after receiving at close quarters a charge of duck shot; but a closer examination will show that in reality all the bones are there, on one side at least. Out of the disjecta membra of the fossil numerous ‘restorations’ have been put together, which are as diverse as the minds which imagined them. We cannot really say with certainty what were the precise relations of the hand to the feathers. It seems most probable that the hand of this ‘mediæval’ bird still retained the ordinary functions of a hand; that it served its possessor to lay hold of convenient branches, from which it fluttered feebly to others. One bold speculator has insisted upon the probability that the Archæopteryx had the requisite five fingers of the presumed ancestral type; but there are no traces of them, except in so far as the lie of the feathers enables a hint to be gathered. Boring operations, or at least prospecting in the interior of the stony slab on which the fossil lies, might reveal some additional fingers; but the operation would be fraught with too obvious perils to a nearly unique object. There are a good many birds which do not, and some which cannot, fly. To the first category belong such birds as the domestic ducks and fowls, and some of the rails. These birds, when put to it--when chased by a dog, for example--can often fly; but as a rule they do not, or at most only flutter along. The Ostrich tribe and a few other birds have totally lost the power of flight. But though this is the case, the bony structure of the hand remains the same in the Ostrich and in the American Rhea; in the Cassowary, however, and the Apteryx of New Zealand, the fingers are reduced to one. The last stage in the atrophy of the organ of flight is seen in the giant and extinct birds of New Zealand, the Moa or Dinornis, in which no trace of a wing has been so far discovered. But in some of these birds in which the wing is reduced in size, or so simplified in structure that it can no longer serve its legitimate purpose, it is made use of for other purposes. When the Ostrich skims along the surface of the sandy deserts where it is often found, it holds out both wings, which are compared to sails; they possibly serve rather as the pole of the tight-rope walker, to preserve the balance of the bird when hurrying along at full speed. In the Secretary Vulture of Africa the wings can be used for flying, but they are also used as weapons wherewith to combat the poisonous snakes upon which the bird so usefully feeds. It strikes down the venomous serpent when the latter is attempting to strike the bird. The Chauna of South America has strong spurs upon its wings, which are used for fighting as well as for flying. But the most curious use to which wings are put is afforded by the Penguin. If the reader has never seen the ‘diving birds’ fed at the Zoological Gardens, let him go there on the first opportunity, and see how rapidly and gracefully the Penguin ‘flies’ under water by the flapping of its wings. They are shorter than those of most birds, and the feathers have become flattened and almost scale-like, so as to offer no resistance to the water; at the same time the bones of the wing are flattened, so that a broad surface is provided, which of course acts like an oar. With this oar-like wing the Penguin can outswim a small fish.

Sternum and Ribs.