The rudders set on the shoulders of the wings are of the strongest feathers, because they feel the greatest strain of all the other feathers.

a b

a b, the rudders of the wings, are employed in the swift descent of the bird. When it wishes to prey and to turn from one place to another without slowing the motion, it uses these rudders; and if these did not exist, it would have to do it with the whole wing, which, being broad, would greatly slow the motion already begun, which would not be its purpose.

Of the oblique motion of birds.

The bird that spends its impetus against the coming of the wind, with wings open and without their movement (except the necessary balancing), if it finds itself above the wind, will always rise, but with so much greater or lesser gain of height as the impetus that moves it is of greater or lesser power, and of lesser or greater obliquity in itself.

But if the bird moves below the wind without beating its wings, then the impetus will be spent in the bird's descent; and the impetus will be the more lasting the less oblique that descent is.

If the bird with wings open and without beating moves together with the wind toward the same aspect, then that bird at every degree of motion will gain a degree of descent. But that descent will be the more oblique the swifter the wind is, as is proved in heavy bodies thrown into running waters.

And if the bird is struck behind and from below by the wind, then the bird will rise on high; but this is very rarely done by birds, because such motion turns their feathers and plumes back toward the head of those birds.