Our Bird Comrades Part 10

All birds by no means possess this particular muscle, but all the perchers have some muscular arrangement in the legs and toes that practically answers the same purpose. If you will bend your wrist backward as far as you can, you will observe that your fingers will have a tendency to curve slightly forward. This is caused by the stretching of the tendons over the convex part of your bent wrist joints.

The typical bird has four digits, three in front and one reaching backward. The hind toe is called the hallux, and corresponds to the thumb of the human hand, so that in grasping an object it can be made to meet any of the other toes. But many birds are not provided with a quartet of digits. The ostrich has only two, the inner and hinder toes being wanting. However, this great fowl does not experience any lack, for its feet are almost solid like hoofs, and quite flat, and hence are especially adapted for traveling across the sandy desert.

No bird has ever been found with more than four toes; and four seem to be ample for all purposes. A fifth toe for a bird would be as useless as a fifth wheel on a wagon. Quite a number of species have only three toes, most of them among the walkers and waders, and none, I believe, among the true perchers. Take the plovers and sanderlings, for example, which spend most of their time, when not on the wing, in running about on the ground, especially along the seash.o.r.e or the banks of streams and lakes, and seldom, if ever, sit on a perch--in their case a fourth toe would be worse than a superfluous appendage; it would be an enc.u.mbrance, dragging along in the mud and mire. In these species it is the hind toe that is lacking, their three digits all being in front, where they are of the greatest service. There is another cla.s.s of birds that have hind toes, though very much reduced because their owners do not perch, but scuttle about on the beach.

This cla.s.s includes the little spotted sandpipers which you often see running or flying along the sh.o.r.es of a river or lake.

Curious to tell, several species of woodp.e.c.k.e.rs are tridactyl--that is, three-toed--and still more curious is the fact that in their case the true hind toe is lacking, while the outer front toe is bent backward, or "reversed," as it is called, and is thus made to do service for a hind toe. The other species of woodp.e.c.k.e.rs have four toes, two in front and two behind, the outer one of the latter pair being a reversed digit. Why some of the woodp.e.c.k.e.rs should have four toes and others only three is an unsolved enigma, and is especially puzzling in view of the fact that the four-toed kinds do not seem to possess any advantage over their cousins. The tridactyl species are as expert climbers as any members of the family, and are extremely hardy birds, too, some of them dwelling the year round in cold northern climates, where the food question must often be a serious one.

[Ill.u.s.tration: Spotted Sandpiper, or "Peet-weet"]

Here is still another conundrum for the bird student: Why do the four-toed woodp.e.c.k.e.rs have two hind digits, despite the fact that they always clamber upward when they take their promenades on the boles and branches of the trees, whereas the agile little nuthatch, which glides upward or downward, as the impulse moves him, has only one rear toe and three in front, like the true perchers? Nor is it less puzzling that the cuckoos, which are perching birds, should have two toes in front and two behind. Then, there is the little brown creeper which never perches and is forever creeping, creeping, upward, upward--save, of course, when it takes to wing--and yet its toes are arranged in the normal percher style, the hind digit having an especially long, curved claw. It is a mistake to suppose that all the problems of the bird world have been solved.

Look at the different kinds of birds' feet and see how wisely they have been planned for the various purposes to which they have been applied.

In order that a bird may use his feet with the greatest dexterity in perching and flitting, his digits should be as free and movable as possible; and so we find that the toes of the perchers are usually cleft to the base, are long and slender, easily opened and closed, and possess the power to grasp an object firmly. The same is true of the raptorial birds, or birds of prey, which are strong perchers and depend largely for their food supply on clutching their victims while on the wing. In all these birds the hind toe is also well developed, and is on the same plane as the anterior digits--a wise adaptation of means to ends.

But there are other birds whose feet, as some one has said, are good feet, but poor hands--that is, they are not intended for prehensile purposes, only for walking and wading. Therefore, in these birds the hind toe is small, and more or less elevated above the plane of the other digits, or, as has already been said, is wholly wanting. The feet of some of these birds are partly webbed, so that, if necessary, they can change their mode of locomotion from running and wading to swimming. Birds whose feet are partly webbed are said to be semipalmated.

This introduces us to that interesting group of birds whose toes are connected throughout their entire length by a thin, membranous web.

Their feet are said to be palmated. We can readily understand why they are thus formed, for their webbed feet answer the purpose of oars to propel them over the water. Most of the swimmers have feet of this kind. Watch them glide like feathered craft over the smooth surface of the stream or lake.

When a swimmer thrusts his foot forward, the toes naturally drop together and partly close, presenting only a narrow front--almost an edge--of resistance to the water; then, when he makes a backward stroke, the toes spread far apart and, with the connecting membranes, are converted into a broad, propelling oar. Is it not a wonderfully wise contrivance?

Most swimming birds have only the front toes webbed, but in a few species, like the pelicans, even the hind toe is connected with its fellows by means of such a membrane. Nor must we forget those water fowls which, instead of palmated feet, have what is called the lobate foot, which means that the digits have broad lobes or flaps on their sides. While in such cases the toes are all distinct, the expanded lobes serve almost, if not quite, as good a purpose for propulsion in the water as do the webs. The coot swims almost as well as the duck or the goose, and at the same time his feet, with their disconnected toes, are better adapted for paddling about amid the watergra.s.s and dense weeds than if they were webbed.

The birds of prey, such as hawks, owls, and eagles, have large, strong, and sharply curved talons and powerful digits, and a sad use they make of them in clutching small birds and animals. The claws of the woodp.e.c.k.e.rs and other climbing birds are stout and extremely acute, just as they should be for clinging to the bark of trees. In short, the structure of a bird's foot, whatever may be the species of fowl, furnishes most conclusive evidence of adaptation in the world of Nature.