Using the sun as a compass requires compensating for its movement across the sky, which birds seem to be able to do by setting their internal biological clocks. One experiment with homing pigeons kept pigeons under artificial light until their biological clock was six hours off from the actual local time. When released, they flew 90 degrees off from the proper direction. Even nocturnal birds seem to take their cue from the sun as cloudy days either result it no migration at night or the migrants show an unusual variation of direction. Stellar Cues .
Cues provided by the stars are obviously important only to nocturnal migrants.
But the most sophisticated experiments put these birds not under the normal night sky, but in a planetarium. This way, the sky could be changed and the birds' response to the moving sky could be measured. In a classic experiment by a German ornithologist, Sauer, in the 1950's, warblers were put into cages in a planetarium in the fall. The next night he did the same thing - move the sky in the direction the birds were oriented. At the end of a couple of weeks, the birds quit showing their restlessness as they had migrated to the night sky that showed middle Africa, where they would have gone if they were free.