Even in this day of scientific research and astuteness, it must not be supposed that everything about the mechanics of avicular flight is understood. We may readily comprehend how a bird, without fluttering its wings, can poise in the air; but how can it move forward or in a circle, and even mount upward, without a visible movement of a pinion? And this some birds are able to do without reference to the direction of the ethereal currents. That, I venture to say, is still a mystery. It almost seems as if some of the masters of aerial navigation in the bird world were gifted with the ability to propel themselves forward by a mere act of volition.

An interesting article on the subject of bird flight appeared not long ago in one of the foremost periodicals of the country, a part of which is here quoted to show what a puzzling problem we have before us:

Recent developments in aerial navigation have renewed interest in the comparative study of the mechanical principles involved in the flying of birds. There is one exceedingly puzzling law in regard to birds and all flying creatures, the solution of which may work far-reaching influences in the construction of flying craft.

"This law, which has thus far perplexed scientists, is that the heavier and bigger the bird or insect, the less relative wing area is required for its support. Thus the area of wing surface of a gnat is forty-nine units of area to every one of weight. In graphic contrast to that, a condor (Sarcorhamphus gryphus) which weighed 16.52 pounds had a wing surface of 9.80 square feet. In other words, though the gnat needs wing surface in a ratio of forty-nine square feet per pound of weight, a great condor manages to sail along majestically with .59 of a square foot to at least a pound of weight. The unexplained phenomenon persists consistently throughout the whole domain of entomology and ornithology. Going up the scale from the gnat, it is found that with the dragon fly this ratio is 30 to 1, with the tipula, or daddy-longlegs, 14.5 to 1, the cockchafer only 5.15 to 1, the rhinoceros beetle 3.14 to 1.

"Among birds the paradoxical law that the smaller the creature the bigger the relative supporting wings holds good. A screech owl (Scops zorca) weighing one-third of a pound had 2.35 square feet of wing surface per pound of weight. A fish hawk (Pandion haliaetus) weighing nearly three pounds had a wing area of 1.08 square feet to each pound. A turkey buzzard weighing 5.6 pounds had a little less than one square foot of wing surface to each pound. A griffon vulture (Gyps fulvus) weighing 16.52 pounds had a wing surface of only .68 square feet to the pound.

"Students of aerial navigation who are devoting much attention to observations of birds say that if the peculiar law governing extant flying creatures could be fathomed the problem of human flight might be solved."



A BIRD'S FOOT

You will agree with me, after you have studied a bird's foot, that it is one of Nature's most wonderful contrivances, so admirably adapted for the purposes to which it is devoted that one cannot help feeling that a Divine Mind must have planned it, just as a man would make a watch for the express purpose of keeping time.

But what is properly included in a bird's foot? Here we shall have to correct a popular mistake, if we wish to be accurate, in the scientific sense of the term. Most people think that the avian foot consists only of the toes and claws, or the part that comes in direct contact with the ground or the perch. That, however, is an error, for the foot really comprises, in addition to the toes and claws, the first long bone of the limb, reaching from the base of the digits to the first joint. You will see, therefore, that the bird walks on its toes, not on its foot as a whole.

The long bone referred to—called the tarsus—corresponds to the instep of the human foot, that is, the foot proper, while the joint which extends backward, forming an angle with the next large bone, is really the bird's heel. Thus you perceive that most birds walk with their heels high in the air. What most people call the bird's "leg" is in reality the bird's foot, and what they call its "foot" comprises only its toes and claws.

To obtain a correct idea of the bird's entire walking apparatus, we begin with the uppermost part of the leg. As we proceed, it would be well to keep in mind the different parts of the human leg and foot. The highest bone is called the thigh bone or femur, which is, for the most part, enclosed in the general integument of the body, and is not entirely separate from it as is the thigh bone of the human leg. Among carvers it is known as the "second joint." It reaches forward and slightly downward, and is hidden under the feathers of the body. The upper end of the femur enlarges into a globular head, which fits into the socket of the hip in the pelvis, while the lower end meets another long bone, which extends obliquely backward and downward and with which it forms the knee joint.

The knee of the bird extends forward, as the human knee does when it is bent. By means of various nodules and tendons the femur is articulated with and fastened to the next large bone at the knee joint. This second bone is the leg proper, called in scientific language the crus. When, with its thick, palatable flesh, it is cooked and placed on the table, it is known as the "drumstick"—a favorite part of the fowl with hungry boys, vying, in their minds, with the "white meat" of the breast.

This important segment of the limb is composed of two bones, the larger of which is called the tibia, the smaller the fibula. At its lower end the tibia forms what is known as the ankle joint by articulating with the next long bone, which is commonly called the tarsus, although the proper name would be really metatarsus. It is not often that this bone is covered with flesh, and therefore it seldom finds its way to the table. Properly speaking, it is the larger part of the bird's foot, reaching obliquely upward and backward from the roots of the toes to the heel. If you will lift yourself upon your toes, holding your heels in the air, you will be able to form a correct idea of what the bird is doing whenever it stands or walks or perches.

The toes are fastened by means of well adapted joints to the lower end of the tarsus, and form what is popularly regarded as the bird's foot. When spoken of separately, these toes are called digits, and when spoken of collectively, they are called the podium. They are composed of small bones called phalanges or internodes, which are jointed upon one another like the several parts of the human fingers. The digits can be spread out for walking purposes, or bent around so as to clasp an object. The outer bone of each digit almost always bears a nail or claw, which is sometimes very strong and hooked, as is the case with the birds of prey, while in other species it is only slightly curved and is not meant as a weapon of offense or defense, but chiefly to enable the bird to "scratch for a living."

How do the birds, in perching and roosting, retain their hold so long on a limb without becoming weary? They do not need to make a conscious effort to do this, but are held by the mechanical action of certain muscles and tendons in the leg and foot. Of course, the bird can also control these muscles by an act of its will, but a large part of their action is automatic. In some species there is a muscle called the ambiens, which has its rise in the pelvis, passes along the inner side of the thigh, whence its tendon runs over the apex of the angle of the knee joint, and down the leg till it joins the muscles that flex the toes. Now when the bird's leg is bent at the joints, as is the case in perching, the tendons of this muscle are stretched over the knee and ankle joints, thus pulling the digits together, and causing them of their own accord to grasp the perch more or less tightly. When a bird wishes to unloose its hold, it simply rises on its feet and relaxes the tendons.

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 seashore 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 encumbrance, 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 class of birds that have hind toes, though very much reduced because their owners do not perch, but scuttle about on the beach. This class includes the little spotted sandpipers which you often see running or flying along the shores of a river or lake.

Curious to tell, several species of woodpeckers 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 woodpeckers have four toes, two in front and two behind, the outer one of the latter pair being a reversed digit. Why some of the woodpeckers 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.



Here is still another conundrum for the bird student: Why do the four-toed woodpeckers 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 watergrass 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 woodpeckers 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.

THE END