The bird's wing changes shape during both the up and down stroke. It also changes its angle of attack depending on how much it presses down and how much it pushes backwards. Flapping flight is basically rowing in the air with the added complication that it needs to generate lift as well. If a Blue Tit stops flapping its wings it better be about to land on a branch or it will fall to the ground. Wing loading is another important way of assessing how and how well a bird flies.
Wing loading is the ratio of wing area to weight.
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