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.