Great Migrations

Great Migrations

Animal migration, however it is defined, is far more than just the movement of animals. It can loosely be described as travel that takes place at regular intervals – often in an annual cycle – that may involve many members of a species, and is rewarded only after a long journey. It suggests inherited instincts. The biologist Hugh Dingle 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 behaviours concerning preparation (such as overfeeding) and arrival; they demand special allocations of energy. And one more: migrating animals maintain an intense attentiveness to the greater mission, which keeps them undistracted by temptations and undeterred by challenges that would turn other animals aside.

An arctic tern, on its 20,000 km flight from the extreme south of South America to the Arctic circle, will take no notice of a nice smelly herring offered from a bird-watcher’s boat along the way. While local gulls will dive voraciously for such handouts, the tern flies on. Why? The arctic tern resists distraction because at that moment it is driven by that instinctive sense of something we humans find admirable: larger purpose. Right now it is totally focused on the journey; its undivided attention is on reaching its destination.

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 partner and right conditions for mating and rearing young.

But migration is a complex issue, and biologists define it differently, depending in part on what sorts of animals they study. Joel Berger, of the University of Montana, argues that migration involves prolonged seasonal round trips between distinct habitats, and that it is the collective nature of these movements that is biologically so significant.

Dingle is an evolutionary biologist who studies insects. His definition is more intricate than Berger’s, citing those five features that distinguish migration from other forms of movement. They allow for the fact that, for example, aphids become sensitive to blue light (from the sky) when taking off on a journey and sensitive to yellow light (reflected from tender young leaves) when landing on host plants. By discovering the biological rules that govern such journeys, Dingle helps guide researchers towards understanding what unites seemingly diverse migrations.

Human behaviour, however, is having a detrimental impact on animal migration. The pronghorn, which resembles an antelope, is the fastest land mammal in the New World. One population of these animals migrates between their summer range in Grand Teton National Park and their winter range on the Red Desert of Wyoming. Pronghorn, dependent on distance vision and speed to flee from predators, traverse open plains to escape deep winter snow in the mountains.

These pronghorn are notable for the invariance of their migration route and the severity of its constriction at three bottlenecks. If they can’t pass through each of the three during their spring journey, they can’t reach their bounty of summer grazing. If they can’t pass through again in autumn, escaping south onto those wind-blown plains, they are likely to die trying to overwinter in the deep snow.

Conservation scientists, along with some biologists and land managers within the USA’s National Park Service, have identified the most critical bottleneck: a narrow corridor between forested hills and private housing developments. Working together with private landholders and federal agencies, they are striving to preserve this passageway so that one of the great terrestrial migrations of North America can endure for generations to come.