15．
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．
59．The author takes squadrons of planes for example to showA．
A．birds may save energy by flying in a V formation
B．birds fly in a V formation for many more reasons
C．planes flying in a V formation follow the leader
D．planes flying in a V formation are more economical
60．Why did Usherwood think it impossible for a bird to fly directly behind another？D
A．Because birds differ from airplanes in wings．
B．Because birds know nothing about aerodynamics．
C．Because birds could hardly create stable currents as planes．
D．Because birds have to overcome great difficulties when flying．
61．What is the scientists'attitude towards the findings mentioned in the passage？A
A．Cautious．
B．Contradictory．
C．Confident．
D．Critical．
62．What would the researchers do in their future studies about the birds？D
A．Calculate the birds'energy savings．
B．Apply the findings to smaller birds．
C．Find how the birds line themselves up．
D．Do research on more common birds．
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