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The arctic tern senses that it can eat later. It can rest later. It can mate later. Right now its implacable focus is the journey; its undivided intent is arrival. Reaching some gravelly coastline in the Arctic, upon which other arctic terns have converged, will serve its larger purpose, as shaped by evolution: finding a place, a time, and a set of circumstances in which it can successfully hatch and rear offspring.

But the process is complex as well as various, and different biologists define it differently, depending in part on what sorts of animals they study. Joel Berger, of the Wildlife Conservation Society and the University of Montana, who works on the American pronghorn and other large terrestrial mammals, prefers what he calls a simple, practical definition suited to his beasts: "Movements from a seasonal home area away to another home area and back again." Generally the reason for such seasonal back-and-forth movement is to seek resources that aren't available within a single area year-round. But daily vertical movements by zooplankton in the ocean—upward by night to seek food, downward by day to escape predators—can also be considered migration. So can the movement of aphids when, having depleted the young leaves on one food plant, their offspring then fly onward to a different host plant, with no one aphid ever returning to where it started.

Dingle, an evolutionary biologist who studies insects, offers a more intricate definition than Berger's, citing those five features (persistence, linearity, undistractibility, special start-and-stop behaviors, stored energy) that distinguish migration from other forms of movement. For example, aphids will become sensitive to blue light (from the sky) when it's time for takeoff on their big journey and sensitive to yellow light (reflected from tender young leaves) when it's appropriate to land. Birds will fatten themselves with heavy feeding in advance of a long migrational flight. The value of his definition, Dingle argues, is that it focuses attention on what the phenomena of the wildebeests and the sandhill cranes share with the phenomenon of the aphids and therefore helps guide researchers toward understanding how evolution by natural selection has produced them all.

Rattlesnake migration on the Great Plains of western Canada is a peculiar but illuminating case. A young Canadian biologist named Dennis Jørgensen, now employed by the World Wildlife Fund, studied movements of the prairie rattlesnake (Crotalus viridis viridis) on the outskirts of Medicine Hat, Alberta, near the northern limit of its range, and found the snakes migrating ambitiously each spring and fall. The average round-trip by his animals was about 5 miles, though an earlier study detected Canadian rattlesnakes migrating as far as 33 miles. In Arizona, by contrast, rattlers don't travel nearly so far, because they don't have to. The driving logic of the Canadian migrations is related to cold winter temperatures (always difficult for reptiles) and the scarcity of really good den sites in which to survive hibernation.

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