On the move

Tens of thousands of species migrate, and the journeys they take are as different as the creatures themselves. Arctic terns migrate from their nesting grounds in the Bering Sea to the Antarctic Ocean, a circumpolar voyage of 35,000 kilometers that may be without equal in the animal kingdom. At the other extreme, spotted salamanders in Maine awake from their winter hibernation in abandoned shrew burrows and trek 135 meters or so across the forest floor to their breeding ponds. Some species of birds and sea turtles appear to follow an invisible roadmap created by the earth's magnetic field. Other animals rely on landmarks such as mountain ranges and coastlines, the alignment of the stars in the night sky, or olfactory cues to determine where they're going. And for plenty of species, we simply don't know how they find their way. What we do know is that migration has become an...