How the bird's wing beats faster than the wind
Raising the wing edgewise with feathers open, then closing them on the downstroke
A full page of inverted mirror-script without diagrams, restating that many birds fly faster than the wind that pushes them because they beat their wings against the carrying wind. Leonardo argues that otherwise birds settled upon the wind could not fly against it. He then describes the wing stroke: on the upstroke the wing goes partly forward and partly up, moving edgewise with each feather turned so that the wing is perforated, and on the downstroke it is driven back against the air so that the feathers and the whole wing close into a solid surface.
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Birds beat their wings against the carrying wind
The flight of many birds is faster than the wind that drives them, because they push their wings against the very wind that carries them. If this were not so, birds resting upon the wind could not fly against it.
Perforated upstroke, solid downstroke
On the upstroke the wing is sent partly forward and partly upward, moving edgewise with each feather turned so the wing is perforated and lets air through. On the downstroke it is thrust back against the air, and the perforation of the feathers and of the whole wing closes into a solid surface.