For a long time people have been trying to build a device for flying like a bird - an ornithopter. The ornithopter is a machine designed to achieve flight by means of flapping wings. The special attention was on that kind of machine able to achieve a sustained flight driven by manual power.
However, designers are attempting to find resolution of the problem of insufficient muscle power, by improving aerodynamic characteristic of designed devices. It is impossible to find a solution that way. I solved that problem by improving mechanical efficiency of the wing drive. In experiments it was shown that enhancing of mechanical efficiency could lowered energy consuming more than 75%.
I believe that improvement opens the opportunity to have a working machine, able to fly like a bird, driving by manual power.
There are more problems with flapping wing driving, but they could be solved by design.