The great mystery
Hundreds of barn swallows sit in rows on telephone wires. Suddenly, with
a great twittering, they all rush into the air. Staying close together
in a big flock, they fly away. It is late summer and the barn swallows
have begun to migrate.
Every autumn millions of geese, ducks, storks, cuckoos, bobolinks, and
other birds migrate. They leave the north, where they spent the summer,
and fly south. Sometimes many birds fly together. Some birds fly alone.
When birds migrate they often fly great distances. Sometimes they cross
oceans and continents. But the birds always go to the same warm parts of
the world where their ancestors have gone for thousands of years. The
birds stay all winter in these warm places. In the spring they migrate
back to the north. Sometimes they go back to the same nests they used
the summer before.
Birds aren’t the only animals that migrate. Monarch butterflies,
ladybugs, and many other insects migrate, too. So do some fish and
mammals.
It is usually still warm when birds and insects begin to migrate. How do
they know winter is coming? How do they know where to go? What makes
them always go to the same place?
Scientists know that something in a bird’s or insect’s body makes it
leave at the right time and steers it the way it should go. But no one
is sure of all the causes. Migration is a great mystery!
