Like moon shots, animal migrations inspire a sense of the epic. Caribou herds hundreds of miles long, millions of monarch butterflies flying across North America each fall, Arctic terns migrating from pole to pole -- all these make us stand in awe of nature and the single-minded determination of its creatures. The tiny blackpoll warbler flies 9,000 miles on its seasonal passage; for its size, this is equivalent to 10 moon trips.
"The Mystery of Migration," an impressive tribute to these epic journeys, catches their primeval poetry. The book is beautifully illustrated, with superbly clear and stunning photos, many in color.
But the thrust of the book is less beauty than science. Its authors are a team of British zoologists led by Dr. Robin Baker, whose work on a possible "sixth," or gravitational, sense in humans has been featured on the "Nova" series on PBS television. Like the shows, the book is meant for a general audience, without jargon or inaccessible material.
Yet the book also pursues its subject with rigor, careful argument, and impressive documentation. It's unusual: not simply a big, beautiful nature book , but one that argues out several relatively new theories as well. A model of both technical and popular scientific writing, "The Mystery of Migration" belongs more in the study than on the coffee table.
One new theory redefines migration more broadly than we might think of it. The authors note that Salmon swim thousands of miles in a "return migration cycle" to their birthplace to spawn and die; yet monarch butterflies rarely return to an exact area, and they spawn in the middle of their migration route. Distance, moreover, is relative; minute zooplankton travel as far as salmon just by floating up and down through several fathoms in coastal seas. The authors argue that there is no single model or set standard for migration, and therefore "the study of migration must embrace all movements from birth to death." The sum of these movements they call the "lifetime track."
"The Mystery of Migration" covers birds, bugs, bats, land mammals, sea mammals, and even plants and humans, treating hunter-gatherers, pastoral nomads, and, less convincingly, immigration, emigration, and commuting. "The lifetime track of the commuter," we are coolly told, "is a daily return migration cycle."
But the most provocative passages on man involve Dr. Baker's recent experiments with the "gravitational" sense. Previous experiments had indicated that birds have this "gravitational" sense. In 1979, a new theory linked it with chemical magnetite, which was discovered in pigeons and which aligns itself with earth's magnetic field. A "gravitational" sense, scientists hypothesize, may allow animals to construct an intuitive map of their longitude and latitude, and navigate accordingly. In his own experiments, Baker found that human subjects, although wearing eye patches and earplugs and displaced over long distances to unfamiliar locales, could still point in the direction of home.
Baker acknowledges that his data are incomplete, but they do provide fascinating reading. Aside from gravitation, evidence indicates that migrating animals use intricate sensory maps and can adjust them when necessary. Some rely on visual landmarks, others use smells, and still others a type of sonar. At night some birds may get their bearings from the sound of the surf on a particular coast.
The evidence indicates that most animals don't have a capacity to reflect on their knowledge or behavior. That is humanity's peculiar mixed blessing. But "The Mystery of Migration" forces us to consider how much these animals do know and how they know it. The explorations here are fascinating but not final. The reader goes away as impressed by migration's continuing magic as by the new theories that partly account for it.