Leonardo studied the flight of birds with his customary zeal, and it is well known that he made designs for a flying machine. Diagrams of constructions like the one above were the result of
exhaustive observation of the movements of birds to examine how the wings achieved flight.
There is a folio of Leonardo's drawings entitled Flight of Birds and in these he seems to have accurately recorded how the flow of air supports the birds, and how their balancing movements depend on the currents of air passing across the wings. In this design, Leonardo has stuck very faithfully to the anatomy of an actual bird's wing.