Beating one wing twice to correct a tilt or to turn
A double beat of one wing rights an over-lowered side and rows the bird around toward a turn
Two notes on asymmetric wing-beats. Leonardo observes that a bird often beats twice with one wing and once with the other when it has sunk too far on that side, using the extra beat to recover. The analogous case is the turn: the bird rows backward twice with one wing while holding the opposite wing almost still toward the place it means to turn. Two small sketches of a bird in flight, seen from above, accompany the text.
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A double beat to recover an over-lowered side
Often the bird beats twice with one wing and only once with the other. It does this when it has sunk too far down on that side. The unequal count of beats works to level the bird again.
Rowing one wing to steer a turn
The bird does the like when it wants to turn to one side: it rows backward twice with one wing while keeping the opposite wing almost still, held toward the place where it means to turn. The still wing marks the pivot while the rowing wing drives the body around.