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.