单项选择题

Pigeons have been used as messengers for 500 years, because of their special ability to find home. The mystery of the homing pigeon is on how it navigates and how it finds home. We now know that there are two ways that pigeons tell directions. First, they use the sun. Just getting rough directions from the sun is easy. However, getting accurate directions from the sun takes more care. To tell direction accurately from the sun, one needs to know the exact time.
All plants and animals seem to have built-in clocks. Usually these biological clocks are not quite exact in measuring time. However, they work pretty well, because they are “reset” every day, maybe when the sun gets up.
Do pigeons use their biological clocks to help them find direction from the sun? We can experiment to find out. We can keep pigeons in a room lighted only by lamps. And we can time the lighting to make their artificial “days” start at some different time from the real outside day. 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.
The above experiment shows that homing pigeons can tell directions by the sun. What happens when the sky is darkly overcast by clouds and no one can see where the sun is? The pigeons still find their way home. So it seems that pigeons also have some extra sense of direction from the earth’s magnetic field when they cannot see the sun.

The secret of the homing pigeons is （）.

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