单选题
Why don't birds get lost on their long migratory
flight? Scientists have puzzled over this question for many years. Now they're
beginning to fill in the blanks. Not long ago, experiments show
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 principle means of
navigation. What do they do when the stars are hidden by clouds? Apparently,
they find their way by such landmarks as mountain ranges, coastlines, and river
courses. But when it's too dark to see these, the warblers circle helplessly,
unable to get their bearings.
单选题
Birds don't get lost on their migratory flights, and the reasons
______.