How the wind acting on the tail turns a bird to face it
The tail as a rudder: a leeward or wind-struck tail swings the bird's front into the wind
Leonardo treats the bird's tail as a rudder acted on by the wind. If the tail lies to leeward, the wind presses the bird down from the middle backward and swings its front to face the wind. If instead the slanted tail is struck from windward, the bird drops in front and again turns into the wind. Small sketches of a bird in flight and a wings-spread bird seen from the front sit in the margins.
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A leeward tail turns the bird's front to the wind
If the rudder, that is the tail, of the bird lies to leeward, the wind presses the bird down from the middle backward. As a result the bird is turned with its front toward the wind. The tail's position relative to the wind sets the whole body's orientation.
A wind-struck slanted tail drops the front
When the bird is struck from windward on the slant of its tail, it drops down in front and turns into the wind. This restates, for the opposite wind direction, the tail's role in swinging the bird to face the airflow.