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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.
The main idea of the passage is that
AMigration over land requires a simpler explanation than migration over water does
BThe means by which animals migrate over water are complex and only partly understood
CThe ability of migrant animals to keep track of time is related to their magnetic sense
Dknowledge of geographic location is essential to migrants with little or no compass sense
Eexplanations of how animals migrate tend to replace, rather than build on,one another
The author maintains that migrating animals would need a fantastic map sense (line10) to determine their geographic position by celestial navigation mostly likely in order to express:
AAdmiration for the ability of the migrants
BSkepticism about celestial navigation as an explanation
Ccertainly that the phenomenon of migration will remain mysterious
DInterest in a new method of accounting for over-water migration
ESurprise that animals apparently navigate in much the same way that human beings do
