The raven, along with a variety of hawks and eagles, flies along the cliff to take advantage of warm air rising along the cliff. Sunlight shines on the rock face, heating it. The rocks heat the air around them, and that heated air expands. That causes the air to be less dense than the surrounding air, so it rises. By soaring through this rising air, the raven can fly for long distances without having to flap its wings. I have seen ravens soar back and forth this way for over half an hour without once flapping their wings.