| Why don't birds {{U}}(71)
{{/U}} lost on their long flights from one place to another? Scientists have
puzzled over this {{U}}(72) {{/U}} for many years. Now they are
beginning to fill {{U}}(73) {{/U}} the blanks. Not long ago, experiments {{U}}(74) {{/U}} that birds rely on the sun to guide them {{U}}(75) {{/U}} daylight, hours. But what about birds {{U}}(76) {{/U}} fly by night? Tests with artificial stars have proved that {{U}}(77) {{/U}} night - flying birds are able to follow the stars in their long - distance {{U}}(78) {{/U}} A dove had spent its lifetime in a cage and had never flown under a {{U}}(79) {{/U}} sky. Yet it showed a(n) {{U}}(80) {{/U}} ability to use the stars for guidance. The bird's cage was placed under a (n) {{U}}(81) {{/U}} star - filled sky. The bird tried to fly in the position as that {{U}}(82) {{/U}} by his outdoor cousins. Any {{U}}(83) {{/U}} in the position of the make believe stars caused a change {{U}}(84) {{/U}} the direction of his flight. {{U}} (85) {{/U}} think that doves, when flying in daylight, use the sun for {{U}}(86) {{/U}}. But the stars are {{U}}(87) {{/U}} their principal means of navigation. What do they do when the stars are {{U}}(88) {{/U}} by clouds? Apparently, they find their way by such landmarks as {{U}}(89) {{/U}} ranges, coast lines, and river courses. But when it's too dark to see {{U}}(90) {{/U}}, the doves circle helplessly, unable to get their bearing(方位). |