A、The weather gets hotter. B、Telegraph wires stand still. C、Swallows sit on telegraph wires. D、People find the mystery of migrat

One of the greatest mysteries of nature is the instinct to migrate. Every year, millions of creatures feel the need to migrate and most people have seen the arrival or departure of migrating flocks of birds. In England, for example, we always know when Autumn is coming, because flocks of swallows sit on the telegraph wires. Migration is not only seen in birds but also in reptiles, for example, turtles, frogs and toads. Insects, such as locusts and butterflies; fish, such as eel, salmon and tunas and mammals, such as reindeer, seals, lemmings, whales and bats. Many of these creatures can find their way over long distances. And how they do it is a mystery and a wonder to everyone, especially for those of us who have difficulty even reading a map covering 100 miles. There are several suggestions about how they navigate. Firstly, it is suggested, they navigate by using stars and planets for guidance. Secondly, some people think that these migrating mammals find their way by following the pole of the earth’s magnetic field. Thirdly some people think that migrating birds, insects and animals follow a certain smell or scent. Fourthly, another group of people believe that animals migrate by recognizing geographical features. Fifthly, some people think that animals migrate by following changes in temperature. And finally, there are people who suppose they migrate by following sound.