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.
[Illustration: Spotted Sandpiper, or "Peet-weet"]
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.