题型:阅读理解 题类:常考题 难易度:普通
江苏省海安高级中学2019届高三上学期英语12月月考试卷
Anyone watching the autumn sky knows that migrating birds fly in a V formation, but scientists have long debated why. A new study finds that these big-winged birds carefully position their wingtips and flap(拍动) at the same rate, probably to catch the upward movement of air and save energy during flight.
There are two reasons birds might fly in a V formation: It may make flight easier, or they're simply following the leader. Squadrons(中 队) of planes can save fuel by flying in a V formation, and many scientists suspect that migrating birds do the same. Models that treated flapping birds like fixed-wing airplanes show that they probably save energy by drafting off each other, but currents created by airplanes are far more stable than those coming off of birds.
Just as aerodynamic(气动力) calculations would predict, the birds positioned themselves to fly just behind and to the side of the bird in front, timing their wing flaps to catch the uplifting eddies(涡流). When a bird flew directly behind another, the timing of the flapping reversed(相反) so that it could minimize the effects of the downdraft coming off the back of the bird's body. “We didn't think this was possible,” Usherwood says, considering that the difficult achievement requires careful flight and incredible awareness of one's neighbors. “Perhaps these big V formation birds can be thought of quite like an airplane with wings that go up and down.”
The findings likely apply to other long-winged birds, such as pelicans and geese, Usherwood says. Smaller birds create more complex wakes(尾波) that would make drafting too difficult. The researchers did not attempt to calculate the birds' energy savings because the necessary physiological measurements would be too invasive for an endangered species. Previous studies show that birds can use 20% to 30% less energy while flying in a V.
Scientists do not know how the birds find that aerodynamic sweet spot, but they suspect that the animals line themselves up either by sight or by sensing air currents through their feathers. In future studies, the researchers will switch to more common birds, such as pigeons or geese. They plan to investigate how the animals decide who sets the course and the pace, and whether a mistake made by the leader can ripple through the rest of the flock to cause traffic jams.
Tiger research: Indonesia While sustainable tourism helps protect many of the Indonesia's wildlife, one of its rarest species, the Sumatran tiger, suffers as the island's cities expand into its forest habitat. Fortunately Biosphere Expeditions have created a trip that not only benefits the seriously endangered animal, but helps protect it too. Based in a remote camp in the heart of the rainforest, volunteers will be needed to tour the landscape, documenting tracks, kills and scats(粪便) to aid important research into the lives of these big cats. Company: Biosphere Expeditions Length: 13 days More information: www.biosphere-expeditions.org |
Cycling tour: Cambodia With an ancient world of temples and jungles, Cambodia is the kind of country that needs to be explored slowly. PEPY Tours offers 621 miles of green fields, dusty orange tracks and winding rivers on its 15-day cycling expeditions. Money goes directly to local people through homestays and lunches, while tours allow visitors to learn Khmer customs and contribute to ongoing community development projects. Company: PEPY Tours Length: 15 days More information: www.pepytours.com |
Beach stay: Tanzania A winner in the Best for Beach category at the Responsible Tourism Awards 2014, Chole Mjini Island is a tourists' paradise(天堂) located off the coast of Tanzania. With no roads or electricity, guests to the low-impact destination can enjoy a simple life in hand-built tree houses and spend days either stretched out on deserted beaches or searching for whale sharks. Company: Chole Mjini Island Length: 6 days More information: www.cholemjini.com |
Teaching: Morocco It's the breathtaking beauty that draws tourists to Morocco's High Atlas Mountains, but while the remoteness of the setting may aid its charm it also causes problems. Few inhabitants of the region's rural villages, especially females, continue their education past primary school. Education For All (EFA) is a Moroccan NGO that builds and runs girls' boarding houses near secondary schools in the High Atlas, making further education theoretically possible. Based in the beautiful town of Asni, the programme is looking for female volunteers with teaching experience. Company: People and Places Length: 4 weeks (minimum) More information: www.travel-peopleandplaces.co.uk |
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