By David Quammen, Photograph by Joel Sartore.
Animal migration is a phenomenon far grander and more patterned than animal movement. It represents collective travel with long-deferred rewards. It suggests premeditation and epic willfulness, codified as inherited instinct. A biologist named Hugh Dingle, striving to understand the essence, has identified five characteristics that apply, in varying degrees and combinations, to all migrations.
They are prolonged movements that carry animals outside familiar habitats; they tend to be linear, not zigzaggy; they involve special behaviors of preparation (such as overfeeding) and arrival; they demand special allocations of energy. And one more: Migrating animals maintain a fervid attentiveness to the greater mission, which keeps them undistracted by temptations and undeterred by challenges that would turn other animals aside. Animal Migration
An arctic tern on its way from Tierra del Fuego to Alaska, for instance, will ignore a nice smelly herring offered from a bird-watcher’s boat in Monterey Bay. Local gulls will dive voraciously for such handouts, while the tern flies on. Why? “Animal migrants do not respond to sensory inputs from resources that would readily elicit responses in other circumstances,” is the dry, careful way Dingle describes it. In plainer words: These critters are hell-for-leather, flat-out just gonna get there.Another way, less scientific, would be to say that the arctic tern resists distraction because it is driven at that moment by an instinctive sense of something we humans find admirable: larger purpose.
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. Animal Migration continues.
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.
http://ngm.nationalgeographic.com/2010/11/great-migrations/quammen-text
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