How Do Birds Navigate a Cloudy Night?

Birds are not very sophisticated at migration, by looking at the sun and the stars we can figure out where north is. Birds like sand hill cranes and water fowl that migrate on that first migration with their parents, have basically been taught. But are they even noticing? Are they picking up oin borden? This is a life or death matter. For birds, if their best, most accurate navigation system is unavailable, they will shift to the next best of their redundant systems.