Part 2

Let us imagine the case of a youth or boy who has read and re-read half a dozen long histories of some one species; and, primed with all this knowledge, who finally goes out to observe it for himself. It will astonish him to find how much he has not been told. He will begin to think that the writers must have been hasty or careless, that they neglected their opportunities, and missed much that they ought not to have missed; and he may even experience a feeling of resentment towards them, as if they had treated him unfairly. But after more time spent in observation he will make the interesting discovery that, so long as they are watched for, fresh things will continue to appear. The reflection will follow that there must be a limit to the things that can be recorded; that the life-history of a bird cannot be contained in any book, however voluminous it may be; and, finally, that books have a quite different object from the one he had imagined. And in the end he will be more than content that it should be so.

W. H. H.

BRITISH BIRDS.

_THE ANATOMY OF A BIRD._

It is very important that every one who studies birds should have some acquaintance with their insides as well as with their outsides. To have a proper appreciation of the mechanism of flight, the most distinctive attribute of a bird, we must explore the air reservoirs and muscles, which combine, with other organs, to form a complicated, but exquisitely adjusted, system. It is true that other animals show a similar adaptation to their several modes of life, but in a bird the necessities of life seem to have produced a more obvious and striking harmony between structure and habit. Furthermore, the young ornithologist should not be content with gaining the ability to recognise the different kinds of birds: he should understand their mutual relations, and the place of a bird in Nature. To form an opinion about these matters needs more than an acquaintance with the colours and outward form, and with the eggs and nest. A great deal can be learnt from these characters, but they are at most only useful in linking together closely related species. All the members of the extensive tribe of parrots, for example, are bound together by their hooked bills, their white eggs, their grasping feet, &c. But we want to go further, and determine what are the relations of the parrots to other birds which differ totally from them in all outward and visible signs. To solve, or rather to attempt to solve, broader questions of this kind we must have recourse to the scalpel, and even to the microscope. Besides, there not only _are_ birds, but there _were_ birds, which have now passed away utterly, leaving behind only a few bones embedded in the rocks. Nothing of an external nature will avail us in considering what these birds were like in their day, and which of existing kinds they most resembled. We must have a knowledge of bones, of osteology, to grapple with the problems which they present. For these reasons I have dealt in the following pages principally with the organs of flight, and with those internal and external characters which are admitted to be of most use in classificatory questions. I have paid less attention to those organs which are not of importance from these points of view.

Feathers and Feathering.

It is only a very few birds that have a complete and continuous covering of feathers. The penguins are in this condition; and some of the ostrich-like birds are so, more than most others. But in other birds the feathers are arranged in tracts, between which are patches of quite, or nearly, bare skin. The technical name for the feathered districts is ‘Pterylia’; that for the bare patches, ‘Apteria.’ If two birds, belonging to different families, are compared, it will often be discovered that they present considerable unlikeness in the mutual arrangement of the feathered and unfeathered tracts. In fact, it was pointed out not far from the beginning of this century that the dispersal of the feathers over the body was one of the very best characters for classifying birds upon. But when the author of this discovery, Professor Nitzsch, of Halle, first published his book on the matter, it was received with some ridicule, and the pictures of birds denuded of their feathers in order to show up clearly the feather tracts were ironically compared to a portion of a poulterer’s shop. This ridicule, however, did not do away with the fact that the character is often of great use in settling the mutual relationships of birds. When a bird is carefully skinned, it will be seen that the feather tracts have their own special slips of muscle inserted into the roots of the feathers. These muscles, when they contract, serve to raise the feathers slightly, and must be of at least subsidiary importance in flying. This is, perhaps, why the feather tracts are so well marked in birds that fly, and explains the reason for their unmarked character in birds that do not. We can easily understand that the movement of the feathers, if the covering were continuous, would be much more difficult and less pronounced than when there were separate patches far enough away from each other to allow of free and independent movement. In the Penguin, which glides smoothly and rapidly under water in pursuit of its fishy prey, a continuous coating of feathers is not only a source of additional warmth, but offers less resistance to the water; so, too, with a running bird like the Emu or Ostrich. But in the case of the latter, at any rate, the young nestling has quite distinct tracts and apteria, thus showing that, although nowadays it is incapable of flight, it has descended from an ancestor that could fly--at least, that is the way in which it is customary to interpret such differences in structure between young animals and their parents. The Apteryx also, of New Zealand, is quite analogous. The old bird has a nearly continuous covering of feathers, but the unhatched young show perfectly distinct patches of feathers with bare spaces between. We shall show on another page that there are other arguments which appear to prove that all these flightless birds have been gradually derived in the course of time from birds that could fly perfectly well. They are an instance, so far, of what is termed degeneration.

The examination of any bird will show that it has several kinds of feathers. They are all constructed upon the same plan, but some are larger than others, and the smallest are soft instead of firm to the touch.

[Illustration: FIG. 1.--SKELETON OF WING OF ARCHÆOPTERYX WITH REMIGES ATTACHED. (Restoration after Pycraft, ‘Natural Science,’ vol. v.)

I, II, III, digits.]

The biggest feathers of all are a set which fringe the wing (see fig. 1) and another set at the end of the tail. These are called respectively the ‘Remiges’ and ‘Rectrices,’ or the ‘rowing’ feathers and the ‘steering’ feathers. Their principal use, as may be imagined, is in flight. The remaining feathers are also to some extent used in flight, but their main use appears to be to keep the body warm. An eider-down quilt, as everybody knows, is the warmest kind of coverlet; the reason being that the feathers are very bad conductors of heat, and do not, therefore, allow the heat of the body to escape. Birds are the hottest of all animals, which is in part due to their covering of feathers. To understand the structure of a typical feather is perhaps a little difficult; but possibly the accompanying figures (figs. 1, 2, 3, 4) will render the explanation easier to follow. The feather consists of a stem which is technically called the rhachis, the word simply signifying stem. From each side of this a row of parallel rodlets arise which are called barbs. These in their turn give rise to another set of processes which are the barbules. This, however, is not all; the barbules are firmly locked together by other processes, so that the entire feather is quite firm, and can be used as a kind of oar with which to row through the air. It does not give when the wings are flapped. The barbules are of two sorts, those nearest to the root of the barb being different from those which are nearest to its tip. The former, as is shown in fig. 2, are shaped something like a knife-blade; they are thickened above and bent in the middle; they gradually taper away to a fine point. Just in the middle, where the bend is, are two or three small teeth (2, fig. 2) on the upper margin. By means of these teeth-like processes the successive barbules are attached to one another. At the end of each barb, as already mentioned, the barbules are of a different structure. A few of them are illustrated in fig. 4. The end is frayed out into a number of delicate spines, of which those farthest from the actual tip are hooked, while those at the tip are only curved and not hook-like. All these spines are called barbicels. They are upon the lower edge of the barbule; but upon the upper edge are a few shorter and stouter spinelets. As the barbules come off in an oblique direction, it follows that each one of them overlaps a considerable number, in fact five, barbules of the opposite barb. The attachment is by these hooklets, or hamuli, as they are usually termed. The stiff feathers which have this elaborate structure are not found at all in the ostrich-like birds; in them there is no need for a firm surface to catch the air; on the contrary, it would be, if anything, disadvantageous to swift runners, as those birds are. The feathers, therefore, are much reduced in complexity, and in some they consist only of the stem and the barbs. Even in flying birds there are plenty of feathers of a simple structure lying between the stronger contour feathers. These are the soft feathers which are generally spoken of as ‘down.’ Some of them are so reduced as to consist of little more than the stem. The same reduction is seen in the wing feathers of the Cassowary. Along the margin of the wing are a few strong black spines, which are really the quills of the wing feathers with no barbs at all; they consist merely of the stem, which has not dwindled in the least, but is quite as strong as it would be in a feather of use for flying. In a good many birds the contour feathers and the down feathers also have a kind of appendix, known as the aftershaft. This is a sort of supplementary feather arising from the stem just at the point where the barbs begin, and having precisely the structure of a small feather. In the Emu and the Cassowary this aftershaft is fully as large as the main feather; from each stem in these birds arise as it were two feathers.

[Illustration: FIGS. 2, 3. (After Wray in ‘Ibis’ for 1887.)

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

[Illustration: FIG. 4.--PORTION OF TWO ADJACENT BARBS. (After Wray in ‘Ibis’ for 1887.)

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.

[Illustration: FIG. 5.--FOOT OF PELICAN.]

[Illustration: FIG. 6.--FOOT OF PERCHING BIRD.]

[Illustration: FIG. 7.--FOOT OF KINGFISHER.]

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.