For those who don't know, the CIA had a similar (unsuccessful) program in the 1960s, which was kept secret for decades:
Their efficiency advantage drops quickly for diminishing sizes and weights.
That is why big birds use gliding whenever they can, but small birds and insects use almost only wing-powered flight.
Screw propellers are pretty much equivalent with wings whose rotation angle is not limited so they can rotate continuously around the body instead of having to reverse periodically the direction of rotation and the attack angles.