Passage One
Why don’t birds get lost on their long migratory flights? Scientists have puzzled over this question for many years. Now they’re beginning to fill in the blanks.
Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly mainly by night? Tests with artificial stars have proved conclusively that certain night-flying birds are able to follow the stars in their long-distance flights.
One such bird—a warbler—had spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird’s cage was placed under an artificial star-filled sky at migration time. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the make-believe stars caused a change in the direction of his flight,
Scientists think that warblers, when flying in daylight, use the sun for guidance. But the stars are apparently their chief means of direction-finding. What do they do when the stars are hidden by clouds? Apparently they find their way by such landmarks as mountain ranges, coast line, and river courses. But when it’s too dark to see these, the warblers circle helplessly, unable to get their directions.

Birds that fly mainly at night follow _____.