British birds

by W. H. Hudson; Frank E. Beddard

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.

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.