Academic Reading — Test 7

Reading passage

The wandering albatross is among the most accomplished long-distance travellers in the natural world. With a wingspan that can exceed three metres, the broadest of any living bird, it ranges across the vast and stormy expanse of the Southern Ocean, often covering thousands of kilometres in a single foraging trip. What makes this achievement so remarkable is not the distance itself but the manner in which it is accomplished. For hours at a time the bird scarcely beats its wings, and yet it advances steadily against and across the prevailing winds. The technique that allows this near-effortless travel is known as dynamic soaring, and understanding it reveals an elegant partnership between an animal and the physics of its environment. Dynamic soaring depends upon a particular feature of wind over open water. Close to the surface of the sea, friction with the waves slows the moving air, so that wind speed is lowest just above the water and increases with height. This vertical change in wind speed is called a wind gradient, and it is especially pronounced in the Southern Ocean, where powerful and persistent westerly winds blow almost without interruption around the globe. The albatross does not fight this gradient; instead it harvests energy from it. By repeatedly climbing into faster-moving air and then descending into slower-moving air, the bird is able to gain kinetic energy from the difference in wind speed between the two layers, much as a sailing boat extracts energy from the wind without any engine of its own. A single cycle of dynamic soaring can be divided into roughly four phases. The bird begins low over the water and turns to climb steeply into the wind, rising into progressively faster air. As it ascends it loses ground speed but the increasing headwind helps to sustain its lift. Near the top of the climb, typically ten to twenty metres above the surface, it turns sharply and levels off. It then descends with the wind behind it, accelerating as it drops back towards the waves. Finally, just above the surface, it turns once more into the wind and the cycle begins again. Each loop carries the bird forward in a long, swooping arc, and by linking many such loops together it can travel enormous distances. Crucially, the energy required to maintain this motion is drawn largely from the wind gradient rather than from the bird's own muscles, which is why the albatross expends so little effort. The anatomy of the albatross is finely adapted to this style of flight. Its wings are extremely long and narrow, a shape that aeronautical engineers describe as having a high aspect ratio. Such wings generate a great deal of lift while producing relatively little drag, making them ideal for gliding but poorly suited to the rapid flapping that smaller birds rely upon. A remarkable feature called the shoulder-lock allows the bird to hold its wings fully outstretched without continuous muscular effort: a sheet of tendon mechanically holds the wing in place, so that soaring for hours costs the albatross little more energy than sitting still. Studies that have attached heart-rate monitors to wild birds confirm that an albatross in sustained gliding flight uses energy at a rate only slightly above its resting level. This mastery of the wind shapes the bird's entire way of life. Because dynamic soaring relies on reliable strong winds, the wandering albatross is concentrated in the windy latitudes of the Southern Hemisphere and is rarely seen in calmer tropical regions, where the technique would fail. The dependence on wind also influences breeding. Adults may travel for days to gather food for a single chick, and the efficiency of their flight makes such distant journeys possible without exhausting the parents. Some tracked individuals have circled the entire continent of Antarctica in a matter of weeks. Yet this same reliance leaves the species vulnerable. Many albatrosses are killed each year on the baited hooks of longline fishing vessels, which they follow in search of an easy meal, and shifts in wind patterns linked to a changing climate may alter the conditions on which their extraordinary flight depends. The wandering albatross therefore stands as both a marvel of natural engineering and a reminder of how closely the fate of a species can be tied to the physical character of its home.