Complex sentence: Especially for takeoff, it would have depended on the wind, as although its legs were strong enough to provide it with a running or jumping start, the wings were simply too long to flap effectively until the bird was some height off the ground.
Split into simpler sentences: Especially for takeoff, it would have depended on the wind. Although its legs were strong enough to provide it with a running or jumping start, the wings were simply too long to flap effectively until the bird was some height off the ground.