完形填空 Science seems to be getting closer to answering a very old mystery．Homing p

完形填空

Science seems to be getting closer to answering a very old mystery．Homing pigeons信鸽)can be 1 hundreds of miles from their homes．When they are let to go to 2 again, they find their way home．Because of this special ability to 3, pigeons have been used 4 messengers for hundreds of years．

Today people even breed homing pigeons for 5 at a sport．The birds are shipped to some chosen 6 a few hundred miles away．Then all of them are let to go together．The winner is the bird that gets home 7．A good race can make it home from 500 miles away 8 a single day．

The mystery of the homing pigeons is how they 9 directions and how they find home．The first part seems to be pretty well 10, and we know of two ways that pigeons tell directions．First, they use the sun．Experiments show that homing pigeons can tell 11 by the sun．What happens when the 12 is darkly overcoat by clouds and no one can 13 where the sun is?Then the pigeons still find their way home．

Naturally, people have wondered whether pigeons might have a built in compass(指南针)－14 that would tell them about the direction of the 15 magnetic(磁的)field．Many different kinds of experiments were 16．Here's what the scientists decided 17 they had made experiments many times．When pigeons can see the sun, they use it as their 18 means of direction-finding．When they can't see the sun, they use some 19 way to sense direction from the earth's magnetic field．

But how do pigeons know 20 direction is toward home?What do they use that we would call a map?These are other questions to be answered．

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)