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  3. According to some scientists, migratory birds should be able to withstand the winter. A bird’s feathery coat is good insulation

According to some scientists, migratory birds should be able to withstand the winter. A bird’s feathery coat is good insulation

admin2022-02-24  8
问题      According to some scientists, migratory birds should be able to withstand the winter. A bird’s feathery coat is good insulation against the cold. Because a bird is warm-blooded, its body temperature always remains constant, even if the temperature of its surroundings changes.
    The factors that trigger migratory behavior in birds are difficult to explain. This behavior seems to be instinctive, not learned. For example many northern species leave their summer homes while the weather is still warm and the food supply plentiful. Young arctic terns born at the arctic breeding grounds will take off with the flock for distant lands they have never seen.
    Bird migrations are probably regulated by the glandular system. Scientists suspect that the changing length of the day is the factor that triggers migratory behavior. In an experiment, migratory birds were kept in artificially lighted rooms. It was found that if periods of darkness were lengthened proportionately, the glands of the birds became active. These glands secrete hormones, which are chemicals that control numerous body functions. Shorter periods of daylight seem to change the hormone balance of birds, so that they retain more fat. This stored fat that provides the energy for a long flight. The same experiment revealed that the birds became more excited as the artificial night was lengthened. It is probably no coincidence that most flocks begin their migratory flights during the night.
What does the passage mainly discuss?
选项 A、Common migratory paths for birds.
B、Why birds migrate.
C、Species of birds that do not migrate.
D、Migration in cold climates.
答案B
解析 文章第二段第一句点明全文主旨，在鸟类不惧严寒的情况下，是什么促使它们迁徙的呢?整篇文章讨论鸟类迁徙的原因。因此，正确答案为B。
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