If the supporting surface of the wing of a bird were made of skin, like the web of the foot of a swimming bird, it would be necessary to fold the wing for each upward stroke. It is here that the structure of feathers adapts itself to the rapid action required for movement in the air. The wing is not one surface but a series of narrow surfaces lapping in such a manner that they unite to form one broad surface when the downward stroke is made, and with the upward stroke are separated so that the air passes between them. Greater power in the downward stroke and less resistance in the upward stroke are also secured by the curvature of the wing. The under side is concave, the upper side convex. Thus in the downward stroke the wing gathers the air under it and so increases the pressure, while in the upward stroke it scatters the air and reduces the pressure. If the wing were equally rigid throughout, the movement of the bird would be mostly upward. The bird in flying moves forward because the front of the wing is rigid and the tips of the feathers, which are directed backward, are flexible. So the air compressed by the wing in the downward stroke escapes backward, and in doing so propels the bird forward. The principle is the same that is applied in the screw propeller of a boat or an airship, except that the wing vibrates while the propeller revolves. The most important function of the tail in flight is to balance the bird. It is of some assistance in steering, but a bird steers its course mostly by manipulation of the wings. =Scratching.= With the exception of the aquatic birds and the ostrich, all the species of poultry belong to the group called by naturalists _Rasores_ or _Scratchers_. Birds of this class have legs of moderate length and very strong, with toes terminating in a stout claw. Normally they have three toes upon which the foot rests when they are standing on a flat surface, and a fourth toe, like a thumb, which assists the other toes to grasp a perch. Some individual birds and some races of birds have the fourth or hind toe double. The leg of a bird is so constructed that when it is bent as the bird sits on a narrow support the toes contract and grasp the support and hold it without any effort on the part of the bird. Thus the bird is as secure in its position on a limb when asleep as if wide awake and looking out for itself.