Passage One Questions 61 to 65 are based on the following passage: Why don’t birds get lost on their long migratory (迁徙的) flight Scientists have puzzled over this question for many years. Now they are 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 (人造的) stars have proved conclusively that certain night-flying birds are able to follow 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 stars for guidance. The bird’s cage was laced 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 artificial stars caused a change in the direction of his flight. Scientists think that warblers, when flying in daylight, use the sun for guidance. But stars are apparently their main means of directed flight in the night. What do they do when stars are hidden by clouds Apparently, they find their way by such landmarks as mountain ranges, coast lines and river courses. But when it is too dark to see these, the warblers circle helplessly, unable to find their way. What does the passage say about warblers’ sense of direction