The bird in its flight without the help of the wind drops half the wing downwards, and thrusts the other half towards the tip backwards; and the part which is moved down prevents the descent of the bird, and that which goes backwards drives the bird forwards.
When the bird raises its wings it brings their extremities near together; and while lowering them it spreads them further apart during the first half of the movement, but after this middle stage as they continue to descend it brings them together again.
When the bird lowers one of its wings necessity constrains it instantly to extend it, for if it did not do so it would turn right over. The bird when it wishes to turn does not beat its wings with equal movement, but moves the one which makes the convex of the circle it describes, more than that which makes the concave of the circle.
Excerpted from Leonard Da Vinci’s Note-Books, arranged and rendered into English, with introductions, by Edward McCurdy. Published by Charles Scribner and Sons, New York, 1906.