Let us assume a bird to be moving in a straight line with a velocity 'u' and a man to be chasing it with a velocity 'v' in the same direction. If v>u in magnitude, then the man will observe the bird to be moving towards him with a velocity 'v-u' in magnitude. However, the bird will still be flapping it's wings towards him. Now as he is moving with a constant velocity (or in other words, as he is in an inertial frame of reference), he should find Newton's laws of motion to be valid. But from his point of view, the action of the wings of the bird and the reaction of the air on the bird appear to be in the same direction. Isn't this against Newton's 3rd law of motion?