Of the underside of the wings. The resisting vanes of the feathers beneath the wings of birds form (b) a cover one over another (a), set behind the flight of the air or wind, so that this air or wind may close the weak vane-part of the feathers down upon the strong opposing vanes of those same feathers.

a b c d

Why the weak vanes lie beneath the strong ones.
The weak vanes of the feathers place themselves beneath the strong vanes, turning their tips toward the tail of the birds, because the air is denser below the flying creatures than above, and denser in front than behind. And the need of flight is that these lateral tips of the feathers not be caught by the striking of the wind, for they would at once be spread and opened one from another and the wind would immediately penetrate them; whence, these vanes being set with their parts curved and their convexity turned toward the sky, the more they are struck by the wind the more they lower and close upon the vanes beneath them with which they are in contact, and so the entry of the wind beneath the lateral fronts of such vanes is prevented.

What quality of air surrounds the birds that fly.
The air surrounding birds is as much thinner above than the ordinary thinness of the rest of the air as it is thicker below, and as much thinner behind than above as the bird's motion is faster in its transverse course than that of the wings toward the earth; and likewise the air is denser in front, at the bird's contact, than below, in proportion to the two said thinnesses of the air.

The straight motion of birds through the air becomes curved toward that side where the wing draws in. And this arises only because every heavy body descends on the side where it has less resistance. Therefore this motion will be called a curvature compounded of a lateral curvature and of a declining curvature, made by the bird upon that which is lateral and is lower than itself.