Each year, traveling by land, air, and sea, animals make time-honored journeys in pursuit of their destiny. They follow paths taken by their forebears, obeying cues they alone perceive and dare not ignore. Swimming, flying, running, and crawling, by day and by night, these creatures venture forth in large groups or solo. If movement is life, ours is truly a planet in motion.
Millions of large hooved animals—wildebeests, zebras, and gazelles—follow circuits hundreds of miles long on Africa's famed Serengeti Plain. Hundreds of thousands of caribou make similar treks over the North American tundra. Arctic terns, weighing less than a pound, fly from the north pole to south pole and back—an excursion of 20,000 to 25,000 miles in all. At the other end of the weight scale, 30-ton gray whales routinely swim 10,000 miles between Baja California and the Bering Sea.
Migration is by no means confined to well-known vertebrate species. Countless tiny, inconspicuous animals migrate as well, as do various plants, fungi, and microbes, often assisted by the wind. Like other natural phenomena, migration is incredibly diverse, yet there are common threads to be found within this intricate web of motion. Many creatures migrate for similar reasons: to seek food or water, avoid bad weather, and find a warm place and otherwise convivial environment to reproduce and raise their young.
Biologists haven't yet settled upon a precise definition of migration. Many researchers insist on round-trip movement—a stipulation that rules out short-lived insects and other minute creatures. Yet the term surely must be broad enough to include monarch butterflies, which fly each year from Mexico to the United States and Canada, leaving the return trip to future generations.
Several features distinguish migrations from more random animal movements, according to Hugh Dingle, an entomologist based at the University of Queensland in Australia. Migration consists of unusually prolonged and directed travel along a relatively straight path that is less susceptible to distractions than everyday wanderings. There are distinct departing and arriving behaviors associated with migration, and animals often make significant reallocations of energy prior to a trip, such as storing a large percentage of their weight in body fat.
While the experts tinker with the definition, the rest of us can marvel at this remarkable phenomenon, played out on the world stage by billions of performers.
Efforts to save whooping cranes, the most endangered of the world's 15 crane subspecies, offer a rare instance of animals learning about migration by watching what human do. Starting in 2001, humans flying ultralight aircraft have led whooping crane chicks on their first migratory flight south from a wildlife refuge in Wisconsin to a newly-created wintering habitat in a Florida preserve--a 1,200-mile journey.