The reaction force of a flying bird is the upward lift by air it experiences when flapping its wings, according to The Physics Classroom. The reaction force by air is in line with Newton’s third law of motion, which states that every action invokes an equal and opposite reaction.
A flying bird applies downward pressure on air when it flaps its wings. The air responds in kind by exerting force in the opposite direction, giving the bird a lift that is almost perpendicular to the wing surface. It is this force that prevents the bird from falling from the sky.