Page 88 - The Miracle of Migration in Animals
P. 88

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





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