Newton's Third Law

It states that for every action there is an equal and opposite reaction.

When birds fly in the air, its wings push air downwards and in the return, the air pushes its wings upwards. The wings of the bird impart action force on air and the air imparts reaction force to the wings. The forces are of the same magnitude but they act in the opposite direction.

Even when you walk, your toes push earth downwards (action force) while an equal and opposite force of the earth pushes (reaction force) your toes back and that is how we can balance ourselves and walk.