What natural movement did Leonardo da Vinci’s ornithopter try to copy?

Leonardo da Vinci’s ornithopter tried to copy bird flight.

An ornithopter is a flying machine designed to generate lift by flapping wings. Leonardo studied birds extensively and produced sketches of machines with wings that a person might operate through pedals, levers, or other mechanical linkages. His goal was to understand and reproduce the motion of a bird’s wings rather than simply imitate the shape of a bird.

Leonardo’s studies recognized that human muscles alone could not easily power wings large enough to lift a person. He therefore explored mechanical assistance and different wing structures. None of his ornithopter designs is known to have achieved sustained human flight, and the machines remained proposals in his notebooks.

The ornithopter is distinct from a glider, which uses fixed wings and gains lift from forward motion, and from a helicopter, which uses rotating blades. Leonardo investigated several types of flying machine, but the ornithopter is specifically defined by flapping-wing propulsion.