For each new theory advanced, tests were per-formed which disproved it. A species of stork when migrating south through the North American plains makes an abrupt turn to the west at approxi-mately Great Falls, Montana to cross the Rockies. But it couldn't be land-marks, because birds cross vast stretches of ocean to remote islands. The golden plover travels over 2000 miles from Alaska to Hawaii over open seas. Some curlews go 6000 miles from Alaska to Tahiti with no landmarks to follow.
Birds use the position of the sun.
Yet many species of birds migrate by night. They then must use the stars. But while it's true that birds rarely start out on a cloudy night, they will continue their migration even if clouds obscure the stars. It is generally acknowledged now that all these factors apply. How they use these things re-mains a puzzle.