16．B 细节理解题.从第一段倒数第三句得知. E The Migration of Birds Here is a scientific experiment on the homing of birds, the fact of which are quite certain. A few years ago seven swallows were caught near their nests at Bremen in Germany. They were marked with a red dye on some of their white feathers, so that they could easily been seen. Then they were taken by plane to Croydon, near London. This is a distance of 400 miles. Then the seven swallows were set free at Croydon. Five of them flew back to their nests at Bremen. How did the birds find their way on that long journey, which they had never made before? That is the great puzzle. It is no good saying that the swallows have a sense of direction. These are just words and explain nothing. We want to know exactly what senses the animals use to find their way, how they know in which direction to go until they can see familiar landmarks. Unfortunately practically no scientific experiments have yet been made on this question. Perhaps migrating birds are the greatest mystery of all. Swallows leave England in August and September, and they fly to Africa, where they stay during our winter. The swallows return to England in the late summer for the south. A lot has been found about the journeys of migrating birds by marking the birds with aluminum rings put on one leg. An address and a number is put on the ring. Swallows from England go as far as South Africa and as many as fourteen birds, marked with rings in England, have been caught again in South Africa. From England to South Africa is a journey of 6,000 miles. And the birds not only return from Africa to England next spring, but often they come back to the nests in the very same house where they nested the year before. 【查看更多】
