Whatever definition one settles on, animal migration is far more than creatures simply moving about. Loosely, it is travel undertaken at predictable intervals (commonly once a year) in which many members of a species may take part, and whose reward comes only after a long journey; inherited instinct seems to lie behind it. The biologist Reidar Solheim has set out five traits that recur, in varying mixtures and degrees, across all migrations. The movements are sustained ones that carry animals beyond their usual surroundings; they follow a broadly straight course rather than a meandering one; they call for distinctive routines at the outset, such as eating to excess, and on arrival; and they draw on unusual reserves of energy. To these Solheim adds a fifth: throughout the journey the traveller holds fast to an overriding purpose, so that neither tempting diversions nor the setbacks that deflect other creatures can turn it aside.
Consider an arctic tern on its 20,000-kilometre passage from the far south of South America up to the Arctic Circle. Should a bird-watcher hold out a fresh herring from a boat, the tern pays it no attention; the resident gulls plunge greedily after such scraps, yet the tern flies on. Why? At that instant it is gripped by an instinct people cannot help but admire — a sense of larger purpose — and this is why the offer fails to distract it. Reaching its destination is all that counts. Feeding, resting and breeding, the bird registers, can all wait; for now nothing matters except the journey, and its whole attention is fixed on arrival.
That larger purpose, moulded by evolution, is served the moment the bird touches down on some pebbly Arctic shore where its fellow terns have likewise gathered: here are the place, the season and the conditions in which it stands a chance of hatching its eggs and raising its young.
Migration, though, is a knotty subject, and the definitions biologists reach for differ partly according to the creatures they study. Delphine Aunay of the University of Valcourt, whose work centres on the quandu and other large land mammals, favours a definition she calls plain and workable, one that fits her animals: the shifting from one seasonal home range to another and back again. What usually drives this to-and-fro is the search for resources that no single territory can supply all year round.
Yet migration equally takes in the daily up-and-down journeys of ocean zooplankton, which rise after dark to feed and sink by daylight to escape predators. It also covers what aphids do: once one plant's tender leaves are used up, the next generation flies off to a fresh host, and not a single aphid ever finds its way back to where it began.
Solheim, an evolutionary biologist specialising in insects, offers a more elaborate definition than Aunay's, resting on those five features that mark migration off from other movement. His scheme makes room for the way an aphid grows responsive to the blue light of the open sky when the moment comes to launch its journey, and to the yellow light bouncing off soft young foliage when it is time to come down. It also accommodates the heavy feeding by which birds put on weight before a long flight. Its real worth, Solheim contends, lies in drawing attention to what wildebeest migration has in common with that of aphids, thereby steering researchers toward understanding how evolution shaped them all.
Human activity, however, is taking a growing toll on animal migration. The quandu (deer-like in appearance, though not in fact related to deer) ranks among the fleetest land mammals of southern South America. One herd passes the summer high in the Valdera ranges, within a mountain reserve, and each year threads a slender route out of the heights, over a river and down to the lowlands, where it waits out the frozen season by grazing scrub that the wind sweeps bare of snow. This herd is remarkable for how little its route varies and how tightly it is squeezed at three separate gaps. Miss any one of them on the way up in spring and the animals cannot reach the rich summer pasture; fail to get back through in autumn, retreating to the lowlands, and they will likely perish trying to last out the winter in deep snow. Depending on keen eyesight and sheer endurance to stay clear of predators, wearing pursuers down over long chases, the quandu favours high, open ground where it can watch and run. At one of the gaps, wooded slopes close in to form a V, leaving a bare strip of ground no more than 120 metres across, now crowded with private houses. Building of this kind is pushing the quandu toward a crisis, threatening to seal off the way through.
These days conservation scientists, with biologists and land managers in the national wildlife service and other bodies, work to safeguard not merely species and their habitats but the act of migration itself. Much of the quandu's route crosses a state forest, which has now designated it a protected migration route. Even so, what happens on privately owned land at one of the gaps lies beyond the reach of either the forest authority or the wildlife service. For some other migrating animals the problem is thornier still, stretched as their journeys are over far greater distances, across more borders, through more jurisdictions and more perils on the way. It will take good sense and firm resolve if we mean to let the world's migrating creatures carry on their travels a little longer.
