Page 88 - The Miracle of Migration in Animals

THE MIRACLE OF MIGRATION IN ANIMALS

Tactics on the Return Journey

Several species of migrating birds make their return journeys in
the spring, but rather than using the route they followed in the au-
tumn, this time they trace more of an ellipse. Some researchers on mi-
gration think that food supply is the basic factor for the difference
between these routes. A related study found that birds unable to find
nourishment when returning by the same route are unable to breed
or even complete their journey.

How Do Birds Find Their Way?

Migrating birds seem to know the weather conditions at the
place where they’re headed and act accordingly. In so doing, they
must take a number of points into account, such as the distance to
their destination, how fast they will have to fly to arrive at the appro-
priate time, and what route to follow.
It’s not yet known what clues birds use to find the location of
their target at the end of migration, thousands of kilometers away, al-
though the research conducted has come up with some guesses. Birds
are thought to use environmental clues, changes in the Earth’s mag-
netic field, the position of the Sun and the stars, and strong winds, as
well as certain smells.
However, recent radar studies have revealed that birds fly far
above the clouds. From that altitude, it’s not possible to see the
ground and therefore, it’s not very likely that migrating birds can
judge which way to go by landmarks. Consequently, birds are
thought to have some structure enabling them to react to changes in
the Earth’s magnetic field, which they use to find their way. Certain
cells in a bird’s brain contain magnetite, a naturally magnetic min-
eral. In various experiments, birds fitted with small magnets showed

86