A bird is at rest on the floor of an air tight box which is being carried by a boy. If the bird starts flying, he feels the box as 

Assuming that the box is sealed, it would make no difference at all. As the bird flies the bird's wings push air downward. As a result of Newton's 3rd Law of Motion for every action there is an equal, but opposite reaction. Therefore, the air pushes upward on the bird enabling the bird to fly. In a closed box, however, this downward moving air collides with the bottom of the box! This means that the force being used to lift the bird is ultimately being supplied by the bottom of the box. No net change.