how do eagles soar
Eagles (and most large birds) fly by soaring; it's much more energy efficient than flapping their wings. We do use the technique for our own flights. The reason eagles and other soaring birds do this rather than flap is that they generally hunt from the air and so spend a lot of time waiting for prey. They don't need to go anywhere specific and don't need to move fast until they spot prey. Until then they want to use as little energy as possible.