Speed of a Bird's Descent by the Rules of Weight
Contracting or spreading the wings and tail governs falling, rising, and alighting
Four short proofs apply Leonardo's rules of weight and motion to a bird's descent: contracting the wings and tail, or descending by a line of least obliquity, makes the fall faster, while spreading them slows it and raises the height of the rebound. Each conclusion is justified by a numbered proposition of gravity, weight, or local motion, arguing that a body occupying less air penetrates it faster. A final note describes how a bird, coming down to alight, raises a wing and flattens half its length to settle gently on its chosen place. Pairs of birds in curved flight are sketched down the right margin.
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Drawn-in wings make the descent faster
A descending bird becomes faster the more it draws in its wings and tail, by the fourth proposition of gravity: the heavy body that occupies less air descends faster. Likewise the bird that descends by a line of lesser obliquity is faster, by the second of gravity, since it takes the shorter path.
Spreading slows the fall and heightens the rebound
The more a bird dilates, the more its descent is slowed, by the fifth of weight. Conversely, the bird that rebounds rises higher the more width it gains, by the fourteenth of local motion, because a body occupying less air along its path penetrates it faster; so the utmost drawing-in of the wings least impedes the rebounding impetus and carries the bird highest.
Settling on a chosen place
When a bird descends over the place where it wishes to alight, it raises its wing on high and flattens half of its length, thereby coming down slowly onto the low spot it has chosen.