Why do birds experience less air drag?

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 Birds have perfectly aerodynamic bodies which enable them to fly properly without experiencing high air drag.

Similarly, the shape of an aeroplane's wings is what makes it able to fly. Aeroplanes' wings are curved on top and flatter on the bottom. That shape makes airflow over the top faster than under the bottom. So, less air pressure is on top of the wing. This condition makes the wing, and the aeroplane it's attached to, move up.