Limitations of bird flight as an aircraft design
The movement of the bird wing during the flight is very complex. Not just the trajectory matters, the shape of the wing itself changes as it moves. Angle of attack changes continuously. It is definitely not just about flapping up and down. For a long time, this limited the possibilities of building such machines, while with recent technology it seems possible.

This complex movement also restricts the maximum speed possible, as even small birds cannot beat the wings more than about 80 times per second. The fastest birds, such as the white-throated needletail, may reach 105 mph (168 km/h) in horizontal flight that would not be impressive for an aircraft.