Here is the question: If birds flap their wings downwards to create an upwards force due to air pressure, how comes and they do not create an equal and opposite downwards force when their wings return to the top position? Th secret lies into the way that the feathers are arranged, which they act as an one-way valve. When the wings flap downwards, the feathers close to make a smooth and air-tight area. On the other hand, when the wings flap upwards, the feathers open and the air can flow through the space between them. Moreover, the wings are fully extended when during the downwards movement to achieve maximum area, whole during the upwards movement they are retracted and bended to minimize the resistance from the air.