A mysterious phenomenon is the ability of over-water migrants to travel on
course. Birds, bees, and other species can keep track
of time without any sensory cues from the outside world, and such “biological
clocks” clearly contribute to their “compass sense.” For example, they can use
the position of the Sun or stars, along with the time
of day, to find north. But compass sense alone cannot
explain how birds navigate the ocean: after a flock traveling east is blown far
south by a storm, it will assume the proper northeasterly course to compensate.
Perhaps, some scientists thought, migrants determine their geographic position
on Earth by celestial navigation, almost as human navigators use stars and
planets, but this would demand of the animals a fantastic map sense. Researchers now
know that some species have a magnetic sense, which might allow migrants to
determine their geographic location by detecting variations in the strength of
the Earth’s magnetic field.
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