What features of the skeleton of birds are associated with their fitness for flight?

Birds are beautiful creatures adapted to flight. Man has long tried to learn to fly, but all attempts were in vain.
Birds have natural features for flying:
1.). Lightweight skeleton.
2.). Lack of teeth.
3.). Special feather device.
The feather consists of a quill (trunk), feathers of the first and second order, crossing, they create a kind of shield that does not allow the wind to get through the feathers and does not allow the bird to fall