Explanation to help get you the answer, not straight-up though:
Both fixed and flapping wings work by deflecting air downwards, says Professor Geoffrey Spedding, Chairman of Aerospace and Mechanical Engineering at the University of Southern California. " [When air is forced downwards] the reaction force is upwards, and this lift force supports the bird's weight. The bird’s wing pushes against the air and the air pushes back with an equal and opposite force. Unfortunately, with every down stroke there must be an up stroke, what goes up must come down. If the bird had an equal upward stroke to its downward stroke, the net force of the bird’s wing flaps would equal zero.
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Hope my knowledge helped you get an answer :3
