The mystery of the homing pigeon is in how it navigates and how it finds home. It may be taken away in a covered-up cage, even a cage that is turned round and round to purposely confuse any sense of direction. To get home, it must fly over country that it has never seen before.
46. Suppose this were to happen to you? What would you need to find your way home (besides a good pair of legs)? I think I would ask for a compass, which always points north, to help find direction. I would also want a map. If a map shows where my home is, then I can use the compass to point me in the direction toward home. Much of the study of homing pigeon leads to the idea that pigeons need the same kind of information as we do. They need to know how to tell direction and they need something like a map to tell which direction is home.
The first part seems to be pretty well answered, and we know of two ways that pigeons tell direction. First, they use the sun. Just getting rough direction from the sun is easy. It rises somewhere toward the east and sets somewhere toward the west.47.Getting accurate direction from the sun takes more care. You need to pay attention to the time of the year. Then you need to watch the path of the sun closely at each hour of the day. To tell direction accurately from the sun, a person needs to know the exact time.
Plants and animals that have been studied carefully (including people) seem to have build-in clocks.48.These biological clocks,as they are called. usually are not quite exact in measuring time. However, they work pretty well because they are “reset” each day, when the sun comes up.
Do pigeons use their biological clocks to help them find directions from the sun? 49. We can keep pigeons in a room lit only by lamps. And we can program the lighting to produce artificial “days.” different from the day outside. After a while we have shifted their clocks. Now we take them far away from home and let them go on a sunny day. Most of them start out as if they know just which way to go, but choose a wrong direction. They have picked a direction that would be correct for the position of the sun and the time of day according to their shifted clocks.
We have talked about one of the more complex experiments that lead to the belief that homing pigeons can tell directions by the sun. But what happens when the sky is darkly overcast by clouds and no one can see where the sun is? Then the pigeons still find their way home. 50.The same experiment has been repeated many times on sunny days and the result was always the same. But on very overcast days clock-shifted pigeons are just as good as normal pigeons in starting out in the right directions. So it seems that pigeons also have some extra sense of direction to use when they cannot see the sun.51. Naturally, people have wondered whether pigeons might have a build-in compass——something that would tell them about the directions of the earth's magnetic field. One way to test that idea would be to see if a pigeon's sense of direction can be fooled by a magnet attached to its back. With a strong magnet close by, a compass can no longer tell direction.
【正确答案】：假设这种事发生在你身上呢?要找到回家的路，除了一双好腿，你还需要什么?我想我会要一个总是指向北方的指南针来帮助我找到方向。
根据太阳获取准确的方向就更需要小心了，需要注意你是处在一年中的什么时间。然后你需要在一天的每个小时密切观察太阳的轨迹。
这些被称之为“生物钟”的时钟，在测量时间时通常并不十分精确。然而，它们用起来却很有效，因为每天太阳升起的时候，这些时钟都会被“重新设定”。
我们可以把鸽子养在只有灯光照明的房间里。我们还可以设定照明时间，人为地制造不同于室外的“白天”。一段时间之后，我们就改变了它们的生物钟。
同样的实验在晴天重复做了多次，结果总是一样的。但在阴云密布的日子里，那些已改变生物钟的鸽子和正常的鸽子一样，朝着正确的方向出发。
自然，人们一直想知道鸽子是否有一个内置的指南针一种可以告诉它们地球磁场方向的东西。验证这一想法的一种方法就是看看鸽子的方向感是否会被附在它背上的磁铁所欺骗。
