And at the risk of confusing people, I thought I'd share the following.
Some sea birds have a rather neat trick called dynamic soaring. It uses wind gradient to stay airborne without flapping their wings. Very neat trick if you can do it.
Close to the sea the wind is going slower than the wind a bit higher up. By climbing through the wind gradient, airspeed increases and they can extract energy. Then turning downwind, they descend through the gradient, increasing airspeed in the dive. Repeat doing linked semicircles and they stay airborne and move along without flapping.
It has been done in a glider, but it's a bit tricky.
Paul