Cole Porter was right. Birds do it. Bees do it. Even bats with fleas do it. 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 that is 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 150 yards or so across the forest floor to their breeding ponds, an annual journey typical of many salamander species. Three-wattled bellbirds in Costa Rica migrate from montane cloud forests to lowland jungles. Like the bellbirds, mountain quail in the western United States retreat from higher elevations during the winter, but they prefer to walk down the mountains rather than fly. Great white sharks will wander halfway across the Pacific Ocean and back over the course of a year, while krill, the little shrimplike crustaceans that are the bread and butter of the Southern Ocean’s food chain, move up and down the water column in response to daylight. Theirs may be a daily migration of only a few hundred feet, but it is nonetheless essential for the survival of millions of other animals, ranging in size from two-ounce storm petrels to one-hundred-ton whales, that either consume krill or eat the creatures that consume krill.
Of course, animals are often on the move, and not all their wanderings fall into the category of migration. There is the daily search for food or shelter. There is the constant patrolling of one’s territory to fend off intruders. And there is dispersal, which is movement away from a given site with no intention to return, as happens when young birds fledge from their nests and seek out their own territories or when fish larvae are carried away from their place of birth by ocean currents. None of these behaviors constitutes migration. Classic migration consists of seasonal back-and-forth journeys between two sites, as exemplified by the springtime reappearance of orioles in the backyard or alewives in the river. Typically, the travel occurs within a generation. In other words, the oriole that leaves New England in the fall will itself, if all goes well, return to New England in the spring. Some insects, however, spread their migrations across generations. Monarch butterflies, for example, leave their wintering grounds in Mexico and fly north in the spring. Upon reaching the southeastern United States, they lay their eggs and die. The caterpillars hatch, develop into butterflies, and continue flying north. They, too, lay their eggs and die. This cycle is repeated for three or four generations until the butterflies have repopulated eastern North America as far north as New England and Canada. The generation that emerges in the late summer in the north then reverses course and heads south to Mexico.
Equally puzzling are the nomadic behaviors of certain birds, insects, and other animals that will depart from one location and wander for hundreds of miles in search of food. They follow no predictable course and appear to have no clear destination. Strictly speaking, such journeys fall somewhere between dispersal and migration. Yet given the distances these nomads travel, the varied habitats they visit, and the hardships they face, it makes little sense to exclude them from a discussion of the plight of migratory species.
The means by which migratory animals navigate from place to place are as diverse as the journeys themselves. Some species follow an invisible road map created by the earth’s magnetic field, which they perceive through tiny magnets in their bodies. Others 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. Some even have a principle guidance mechanism and one or more backup systems— redundancy analogous to the backup systems on commercial jets. Thus, on clear evenings, a migrating bird may navigate based on the apparent rotation of the stars, while on cloudy nights it can use the earth’s magnetic field. For plenty of species, however, we simply don’t know how they find their way. Yet somehow they manage to sniff, see, or sense when to go, where to go, and when to return.