You should spend about 20 minutes on Questions 14–26, which are based on Reading Passage 2 below.
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, may involve many members of a species, and is rewarded only after a long journey. It suggests inherited instinct.
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 zigzagging; 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 smelly herring offered from a bird-watcher’s boat along the way. While local gulls will dive for such handouts, the tern flies on. It resists distraction because it is driven by an instinctive sense of larger purpose. The bird senses that it can eat, rest and mate later. Right now it is focused on the journey. Reaching gravelly coastline in the Arctic will serve its larger evolutionary purpose: finding the right place and circumstances in which to hatch and rear offspring.
But migration is complex, and biologists define it differently depending on what sorts of animals they study. Joel Berger, who works on the American pronghorn and other large terrestrial mammals, prefers a practical definition: movements from one seasonal home area to another and back again. Generally this movement is to seek resources that are not available within a single area year-round.
Daily vertical movements by zooplankton in the ocean, upward by night to seek food and 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 fly onward to a different host plant, with no one aphid ever returning to where it started.
Dingle, an evolutionary biologist who studies insects, gives a more intricate definition. His five features allow for the fact that aphids become sensitive to blue light from the sky when it is time for takeoff and sensitive to yellow light reflected from tender young leaves when it is appropriate to land. Birds will fatten themselves with heavy feeding before a long migrational flight. Dingle argues that his definition focuses attention on what wildebeest migration shares with aphid movement, helping researchers understand how evolution has produced them all.
Human behaviour, however, is having a detrimental impact on animal migration. The pronghorn, which resembles an antelope though they are unrelated, is the fastest land mammal of the New World. One population spends summer in the mountainous Grand Teton National Park in the western USA, then follows a narrow route across a river and down onto the plains. There they wait out the frozen months, feeding mainly on sagebrush blown clear of snow.
These pronghorn are notable for the invariance of their migration route and the severity of its constriction at three bottlenecks. If they cannot pass through each bottleneck during spring migration, they cannot reach their summer grazing; if they cannot pass through again in autumn, escaping south onto the windblown plains, they are likely to die trying to overwinter in deep snow.
Pronghorn depend on distance vision and speed to keep safe from predators. They traverse high, open shoulders of land, where they can see and run. At one bottleneck, forested hills form a V, leaving a corridor of open ground only about 150 metres wide, filled with private homes. Increasing development threatens to choke off the passageway.
Conservation scientists, biologists and land managers are now working to preserve migrational behaviours, not just species and habitats. A National Forest has recognised much of the pronghorn path as a protected migration corridor. But agencies cannot control what happens on private land at a bottleneck. With other migrating species, the challenge is complicated further by greater distances, more jurisdictions, more borders and more dangers along the way. Wisdom and resoluteness will be needed to ensure that migrating species can continue their journeys.