When animals make long journeys across
places where there is no food or shelter, such as deserts or oceans, it is very
important that they should be able to find their way accurately.
Birds find their way by the stars at night and by the sun
during the day. People thought that this was impossible. Then some scientists
made an experiment. They put some migrating birds(候鸟)in cages inside a
planetarium(天象图). When the operator turned the artificial sky(人造天空)round, the
birds began to fly to the sides of the cage. Every time he moved it, the birds
moved to a new position. Scientists discovered that they always flew toward the
direction in which they would have flown if the sky had been real. This proved
that they could see the stars and respond to them. Many
animals, especially birds, have a very precise sense of time, which is called
their “internal clock”. In cloudy weather, birds delay setting off on long
journeys, but if the cloud lasts for a long time, they must go at last, to
complete their migration in time. In such conditions, they are able to
steer(引导)by following the magnetic field(磁场)of the earth. We could say that they
have their own special compass inside them, which tells the right direction to
go. Most migrating animals travel in groups, sometimes in
very large numbers. This makes it more likely that they will find the right way,
especially when the group has experienced animals which have made the journey
before. |