The Zoologist/3rd series, vol 1 (1877)/Issue 5/On the Anatomical Characters Distinguishing the Swallow and the Swift

by Alfred Henry Garrod

 

ON THE ANATOMICAL CHARACTERS DISTINGUISHING
THE SWALLOW AND THE SWIFT.

By A.H. Garrod, M.A., F.R.S.

 

It is not a difficult task, whatever the department of the subject which may be under consideration, to classify thinking naturalists in one or other of two divisions, dependent upon the nature of the facts which they, from opportunity or inclination, are in the habit of specially emphasising. There are those who devote themselves to the study of the animal as a whole, its external conformation, its habits, and its haunts. They collect specimens of allied species and preserve them according to an arrangement which is liable to be modified by the experience of collectors generally, and from rumours which reach them as to the results arrived at by those of the class to be referred to immediately. These latter lay but little stress upon superficial resemblance and specific differences, devoting their attention to those facts brought to light by osteological comparison or the differences of deep-seated soft parts, which throw light upon the mutual relations of the larger groups into which the subjects of their study are generally acknowledged to fall.

Now and again the opinions of these two classes of naturalists are apt to be diametrically opposed. The one, as the result of his experience that intimately allied forms closely resemble one another, is apt hastily to draw the illogical conclusion that the converse of the proposition is equally true, and that therefore animals which closely resemble one another in contour and habit must be very nearly related. The other, basing his conclusions on different data, does not run the risk of being misled by the deceptive argument, and forms an opinion which has therefore the average value of his productions.

As an excellent example of the above-mentioned opposed notions of naturalists, the relation of the Swallow to the Swift stands prominently forward. By systematic ornithologists and collectors of birds until recently it has always been the habit to place these birds in juxtaposition, contrary to the opinion of anatomists, just in the same way that the Sand Grouse is grouped by them with the Tetraonidæ, the Petrels with the Gulls, and the Secretary Bird with the Caracaras.

Let us look into the subject a little more closely. The common Swift and the common Swallow are birds which intimately resemble one another in many respects. Their size and general coloration are much the same. In both the beak is very broad and short; the first bone of the pointed wing, which corresponds to the human upper arm bone, being also particularly short; whilst the bones of the wing which agree with those of the fore arm—the radius and the ulna—are proportionately very long. In both the feet are small, and the power of progression on the ground feeble, each living almost entirely on the wing, making the smaller insects its staple article of food, and each building its nest in walls or eaves of roofs, not in the branches of trees.

This collection of external resemblances would generally be accepted as sufficient evidence that the Swallow and the Swift are closely allied birds; in other words, that in the pedigree of the bird-class they sprang from a common ancestor, at some, zoologically speaking, comparatively recent time. Further, the fact that the two birds are described next to one another, or placed side by side in collections, by many of those who are in the habit of employing a systematic method of arranging the different genera, would show that such ornithologists consider the relationship between the Swallow and the Swift to be more intimate than that between either of these birds and the Sparrow, Crow, Starling, Lark, &c. But all these last-named birds are what are known as Passerine; in other words, they possess certain anatomical peculiarities in their organization, found in them all, and in no other group of birds. If, therefore, the Swift and the Swallow are more nearly related to one another than either is to any other passerine bird, then, as the Swallow is most certainly passerine, the Swift must be so also.

But certain naturalists assert that the Swift is not a passerine bird at all, and, if they are correct, it is evident that the Swallow and it cannot have anything to do with one another. Upon this assumption, therefore, the passerine Swallow is much more closely related to the Sparrow, the Crow, and the Lark than it is to the Swift.

The question then presents itself—Is it really the case that the importance of the deep-seated anatomical resemblances between the Swallow and the Sparrow, and of the differences between the Swallow and the Swift, is sufficient to justify us, notwithstanding the external similarity between the last-named birds, in believing that the first-mentioned are truly more intimately related the one to the other?

It may be worth while taking a rapid glance at what some of these most important anatomical resemblances and differences happen to be. One of them is the manner in which the feathers are arranged on the skin. Most of us know that, unlike the hair upon a cat or other quadruped, the feathers of a bird are not uniformly distributed over the surface of the body, but grow in linear clusters called tracts, with naked intervals, termed spaces, between them. This may be readily verified by plucking, say a Sparrow, and noticing the thick and opaque light-coloured bands formed by the thickening of the skin surrounding the holes out of which the feathers have been extracted. Between these tracts the skin is seen to be thin and translucent, forming naked spaces through which the colour of the underlying muscles is apparent.

The careful study, some five and forty years ago, by the eminent German ornithologist, C.L. Nitzsch, led him to the conclusion, among others, that these feather tracts are arranged upon a very different plan in the Swallows to what they are in the Swifts, whilst in the Sparrows and their allies they very closely resemble the Swallows. Further he showed that in this feature the Swifts and the Humming Birds are almost identical.

Again, the breast-bone or sternum in birds is much expanded to give origin to the powerful muscles of flight. In both the Swallow and the Sparrow, as in passerine birds generally, its usually oblong figure is modified by the presence of two deep notches, one on each side of the keel, in the posterior margin. But in the Swift there are no such notches to be found, the posterior margin being entire, and in other respects it differs from the same bone in the Passeres, whilst in all it resembles the Humming Birds.

In the Sparrow and the Swallow, again, as in the great majority of the passerine birds, there is at the lower end of the trachea or windpipe, where the bronchi which place it in communication with the lungs arise, an elaborate special mechanism which is known as the muscular organ of voice or lower larynx, by which they have the power—although they do not all employ it—of modulating their note so as to produce a song: this is not found in the Swifts.

In man the greater part of the alimentary canal is composed of a tube of small diameter—the small intestine—which is continued onwards as a more capacious one, the large intestine. These two are not simple continuations one of the other, but the former enters the latter obliquely, the nearer end of the large intestine remaining free as the "blind gut" or cœcum. In the Swallow and Sparrow, as in all the Passeres, instead of there being a single cœcum at the place of junction of the two intestines, there are two. These are not found in the Swifts, nor in the Humming Birds.

In the Swallow, the Sparrow, and all their true allies, it is always the case that the tendons which contract up the last joints of the toes are so arranged that the birds have the power of folding the toe which corresponds to our great toe (the one directed backwards) without moving any of the others. In the Swift, however, whenever the great toe (the hallux) is fully flexed, it is impossible that the other toes should remain opened out, because the two muscles which act on one and the other are bound together by a tendonous band.

In the Swallow, the Sparrow, and most singing birds the number of feathers in the tail is twelve. In the Swifts and Humming Birds the number is always ten—another important difference.

In the Swallow also, as in all the passerine birds, there is a slender muscle running through the thin triangular membrane of the wing between the arm and the fore arm, which is quite peculiar in the manner of insertion or attachment, no other birds possessing the same arrangement. In the Swift this muscle terminates in quite a different manner, here again resembling the Humming Birds exactly.

Taking these several characters into consideration, and realizing how little they are susceptible, on account of their deep-seatedness, to the influence of slight external changes in the mode of life of the species, we are inevitably driven to the conclusion that their weight is overwhelmingly greater than that of the superficial similarity which is so readily brought about by the similarity of the circumstances under which the two species are accustomed to live, and that the resemblances between them are, so far as their constitutions are concerned, dependent only on the fact that they both have—with different pedigrees—arrived at a superficial similarity in contour because they subsist exclusively on the same food.