Dopigeons use their biological clocks to help them find directions from the sun? We can keeppigeons in a room lit only by lamps. And we can program the lighting toproduce artificial"days", different from the day outside. After awhile we have shifted theirclocks. Now we take them far away from home and let them go on a sunnyday. Most of them start out as if they know just which way to go,but choose a wrongdirection. They have picked a direction that would be correct for the position ofthe sun and the time of day according to their shifted clocks. Wehave talked about one of the more complex experiments that lead to the belief thathoming pigeons can tell directions by the sun. But what happens when the sky isdarkly overcast by clouds and no one can see where the sun is? Then the pigeons stillfind their way home. The same experiment has been repeated many times on sunnydays and the result was always the same. But on very overcast days clock-shiftedpigeons are just as good as normal pigeons in starting out in the right directions. Soit seems that pigeons also have some extra sense of direction to use when they cannotsee the sun. Naturally,people have wondered whether pigeons might have a build-in compass-somethingthat 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 befooled by a magnet attached to its back. With a strong magnet close by, acompass can no longer tell direction.
changed slightly (Para. 1)
【正确答案】：shifted
