British birds

by W. H. Hudson; Frank E. Beddard

Fig. 14.—Gizzard of Swan.

o, orifice of duodenum; a, end of proventriculus; cd, muscular part of gizzard.

The gizzard (fig. 14) of the fowl is simply a part of the stomach which has especially hard and muscular walls, the other half remaining soft in texture; this latter is termed the proventriculus, and into it open the mouths of glands which secrete the digestive juice of the stomach. But the muscular part of the stomach—the gizzard—has to grind down the frequently hard food of the bird, so it has not merely a strong wall made of muscle, but also a very tough lining; the whole organ, therefore, forms a highly efficient mechanism for crushing and grinding the seeds and other hard vegetable food which is swallowed. It is rendered more useful still for this purpose by the pebbles which every bird takes care to swallow. The true and singular stories about the varied contents of an Ostrich’s stomach are founded upon the fact that, like other birds, it picks up stones, and with them occasionally other objects. But all birds do not possess a hard gizzard; in Hawks and fish-eating birds the walls are thinner, and the organ is flaccid instead of being rigid. By a very curious and unique exception certain Tanagers, a race of large, often bright-coloured, American, finch-like birds, have nothing at all that can be compared to the gizzard of other birds; this part of the alimentary canal is totally wanting. Now the difference between the gizzard of the grain-eating fowl and the flesh-eating hawk is chiefly a matter of diet. The celebrated anatomist, John Hunter, who lived in the last century, and wrote so much about the anatomy of all kinds of animals, including birds, found that he could feed a soft-stomached bird into one with a hard gizzard, and vice versâ.

We can pass briefly over the rest of the alimentary system, which does not vary a great deal in different birds. The intestines are always rather short, and are diversely coiled, the method of coiling being often characteristic of a particular group. A good way down the intestine are a pair of cæca, which may be entirely absent, as in the Hornbills, for example; and if present may be extremely short, as in the Sparrow, or very long, as in the Ostrich; various intermediate degrees exist. As in all vertebrated animals, two glands pour their secretion into the intestine; these are the pancreas and the liver. The secretion of the liver is the bile; this fluid is accumulated as it is formed in a largish bag—the gall-bladder, in those birds which possess one. Shakespeare used the epithet ‘pigeon-livered,’ which meant literally the absence of a gall-bladder; but, oddly enough, there are some kinds of pigeons which have a gall-bladder, while others, like the common pigeon, have not. The intestine ends in the cloaca, which is the common chamber into which the urinary and generative organs also open.

Tongue and Teeth.

In the inside of a bird’s mouth we find only one of the two things that we might expect to find: there is a tongue, but no teeth. We shall come back to the teeth immediately. The tongue is not so useful among the majority of birds as it is in most mammals. But some do make use of it to a great extent. If you watch a parrot eating its food, you will observe that its thick and fleshy tongue is of the greatest assistance in helping it to manipulate the pieces of food—to extract, for instance, the kernel from a seed or nut. It plays exactly the same part as it does with us. In one kind of parrot, called the ‘Brush-tongued Parakeet,’ the tongue is frayed out at the free end into a brush-like extremity. And there are some small birds, which peck at flowers and live upon honey, in which the tongue is thin and delicate, and frayed out in the same way; this allows them to suck up the juices of the flower. In the Hummingbird the tongue is rolled up so as to form two tubes running side by side, and the same power of sucking up juices is acquired by this means, which, curiously enough, is exactly paralleled by the proboscis of the butterfly. In other birds the tongue is sometimes merely a thin, flat, horny projection, and in others, again, it is just not absent altogether.

A little reflection about the habits of birds will show that they really do not want teeth; and we know that Nature is a most rigid economist: nothing superfluous is allowed in the body. Even rapacious birds like Owls and Hawks have no teeth, because they have a powerful beak and claws, with which the food may be as effectually torn to pieces. Birds such as the Pigeon, which feed upon grain, possess a gizzard—which we have had something to say about already—that performs effectually the function of a mill, grinding into a powder the hard grains of wheat and other seeds which the bird swallows. Nevertheless birds once did possess teeth. In earlier times of the history of this earth there were some birds whose jaws had as formidable a range of teeth as the mouth of many reptiles. They were fish-eaters, and have been named Hesperornis and Ichthyornis. The first was something like a Diver in shape, the latter more like a Gull. A still more ancient bird, the oldest form of bird known to us, the Archæopteryx, had also toothed jaws. In fact, in the old days it was the rule for birds to have teeth, whereas now it is the rule, without a single exception, for birds to be toothless. Perhaps these ancient and extinct forms had some corresponding disadvantage when compared with their modern representatives; their teeth and claws, for example, may have been less effective. But although there is no bird now living which has real teeth, traces of these organs have been discovered in the young embryos of certain birds, which seems to be an absolute proof that they, at any rate, had for their first parents toothed birds. But although modern birds have no teeth, with enamel, dentine, and so forth, all complete, the horny beak has occasionally ridges which to some extent play the part of teeth. The inside of the Duck’s mouth is rough with such ridges, which occur also in some other birds. The large Flamingo was for some time regarded as a long-legged and awkward Duck that had partially adopted the habits of a Stork, partly on account of the fact that the inner edges of the beak were ridged in a fashion exactly like that of the Duck. But it happens that there is a Stork, a true Stork, in India, whose scientific name is Anastomus, which has similar ridges. Ducks feed to some extent upon shellfish, which the roughened edges of the beak are well suited to crush. The replacement in the course of ages of true teeth by horny teeth is seen—a curiously parallel case—in the Duck-billed Platypus of Australia, which has when adult horny plates instead of teeth, but when young has real teeth.

Heart.

As with all vertebrated animals, birds have a centrally placed heart, with which are connected arteries and veins, the two systems of tubes being connected at the ends farthest away from the heart by minute vessels—the capillaries. In relation, no doubt, to the intelligence and activity of birds, as compared with their slower relatives, the reptiles, we find a heart of much more perfect organisation. There are four distinct chambers, as in the mammal, so that the arterial and venous blood are separate, and do not commingle. The two sides of the heart are only in indirect communication by way of the arteries and veins and capillaries. The left ventricle gives rise to the aorta, which is the great arterial trunk of the heart; this divides into the carotid and other arteries, which supply the entire body, with the exception of the lungs. The blood, which is sent out through this vessel by the contractions of the ventricle, permeates the system generally, and is then collected into a series of veins, which ultimately unite into two great veins, the venæ cavæ in front, and a large vein situated posteriorly, the inferior vena cava. These pour the blood back into the right auricle, whence it passes at once to the right ventricle. From the right ventricle it is driven into the lungs, whence it is returned to the left auricle, and so into the left ventricle to renew the circulation. The two chambers of each half of the heart are guarded from each other by valves, which only allow the blood to flow in the proper direction, as stated in the above brief description of the course of the circulation. It is a curious fact that the valve which separates the right auricle and ventricle is a completely muscular structure, while the other is membranous. Moreover, it does not form a complete circle, but is deficient upon one side of the orifice. The interest of this fact is not merely in its abnormality, its divergence from what one would expect, but in the resemblance which is thus shown to a group of mammals, the Monotremata. This group includes only the Duck-billed Platypus of Australia and the spiny Anteater (Echidna) of the same continent and New Guinea. In both of these animals the heart valve in question is also largely muscular, and does not entirely encircle the opening from the auricle. These two mammals also, as everyone knows by this time, have the strange habit for a mammal of laying eggs, which is one among some other reasons which once led naturalists to place them in the neighbourhood of birds. The egg-laying, of course, is not distinctive, since reptiles have the same way of bringing forth their young; and as to the heart valve, it is rather to be explained by the fact that both types of animals are low in the scale of their respective groups, and therefore both approach a common ancestral form.