The wing's thumb as a rudder in flight
The thick finger of the wing resists sideways sliding; folded wings fall faster
Leonardo describes how a soaring bird uses the thick 'finger' (thumb) of its wing: by making it tremble it keeps itself from descending, and by turning these finger-rudders to face the air it resists sliding sideways down its own slope. A second note observes that a bird descends the faster the less air it grasps between the tips of its wings. Sketches show a bird with raised wings and silhouettes of wings held level or folded downward.
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The wing's thumb resists sliding
The bird uses the thick finger of its wings while it flies and holds itself aloft: by making them tremble it keeps from descending. These finger-rudders are also turned to face the air toward which the bird's tilt would make it slide, and by striking against that air they resist the sideways slip.
Less air grasped, the quicker the fall
A short note states that the bird which takes hold of less air between the extremities of its wing tips descends the more quickly. It links the amount of air 'grasped' by the outstretched wings to the rate of descent.