Why do birds need legs if they can fly?
There is a seabird that has adapted so thoroughly, it is effectively purpose-built. Through lack of use its legs have become too small to support its weight. To save weight, its ability to create oil and become waterproof has been left behind. As such, though it is a seabird, if it ever lands in water it will drown. It can and must stay in the air above the ocean for months at a time. Even when it sleeps, it rests still in motion on the wind.