Why migrating flocks fly against the wind
Birds await favorable winds; thrushes' weak feathers force flight into the wind; incident and reflected bounds
Leonardo explains that before long migrations birds wait for favorable winds, and that species differ in how they take them: some fly by jerking bounds, some receive the wind on one flank, some from every direction. Birds that fly in jerking bounds, such as thrushes flying in flocks, have weak wing-feathers poorly armed by the smaller covering feathers, so they must fly against the wind, which presses their feathers smooth; a following wind would ruffle them, throwing the birds into confused, leaf-like tumbling and leaving their flesh exposed to the cold. He describes their bounding flight, a descent with wings drawn in under the wind and a reflected rise with wings open above it, and a marginal note adds that drawing in the wings makes the bird heavier while opening them makes it lighter. A diagram of birds tracing undulating flight-paths, not included in the transcription, runs across the top of the sheet.
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Why thrushes and flocking birds fly against the wind
Thrushes and similar flocking birds have weak wing-feathers poorly armed by the smaller covering feathers, so they must fly against the wind, which closes and presses one feather on another, making the surface clean and slippery. A following wind would enter under each feather, turn it back toward the head, and throw the flight into confused, leaf-like whirling.
Bounding flight: incident descent, reflected rise
To avoid a following wind the birds fly against its course with an undulating motion, their bounds gaining great impetus in the descent, made with the wings drawn in under the wind. The reflected motion, made with the wings open above the wind, returns the bird to the height from which it first descended, and so it continues to its destination.
Wings drawn in add weight; opened wings add lightness
In the incident motion the bird draws in its wings, and in the reflected motion it opens them. It becomes heavier the more it draws in its wings and lighter the more it opens them; the reflected motion is always made against the wind, the incident in the direction the wind moves.
Undulating flight-paths drawn across the top
Across the head of the sheet birds are sketched tracing scalloped, undulating trajectories through hatched lines suggesting the wind, matching the bounding, up-and-down flight described in the prose. This drawing is not covered by the page's transcription.