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SAN FRANCISCO--- As skies are filled with millions of migrating birds, Europeans scientists say the seasonal miracle appears to depend on a seeming contradiction: The fatter the bird, the more  efficiently it flies.
The result of their study contradict a central theory of aerodynamics(空气动力学), which predicts that the power needed to fly increases with weight.
For birds, apparently, the cost of flying with heavy fuel loads is considerably smaller than previously thought.
Researchers found that red knot wading birds double their normal body weight of 100 grams before making their twice-a-year, non-stop commute(路程) between the British Isles and the Russian Arctic. Distance: 5,000 kilometers.
Another study in the journal Nature measured the benefits of flying in an aerodynamic V formation, which allows bird to save energy by gliding in the lead bird’s air stream.
Flying in formation, their heart rates were as much as 14.5 percent lower than flying solo, according to Henri Weimerskirch, a French scientist. The findings help explain how birds complete difficult migrations.
Researchers had assumed that thinner, more athletic birds would have the best chance of survival.
The first study suggests that building up fat deposits(存放) to be burned as fuel during the migrating is worth more than the energy it takes to carry the additional weight. In the study, researchers said their team studied the birds flown at different body masses during 28 simulated(模拟) flights. They were injected with a small amount of water containing a radioactive element that enabled the team to measure the amount of energy burned.
63. Researchers used to believe_______.
A. the thinner a bird is, the less energy it needs to fly
B. migratory birds make a journey from the British Isles to the Russian Arctic
C. aerodynamics makes no sense
D. birds eat more before they begin their migrations
64. According to the passage, birds prefer to fly in the V formation rather than fly solo because_____.
A. it is against aerodynamic to fly in the V formation
B. they can save energy while migrating 
C. they won’t get lost with a bird leading the way
D. in this way their heart beat faster so that they can fly faster
65. The researchers didn’t ______ in the study.
A. inject the birds with water containing a radioactive element
B. watch birds of different body masses in simulated flights
C. take the birds’ heart rates   
D. feed the birds to fatten them
66. The best title of the passage should be _______.
A. Birds Fatten up for Journey        B. Migratory Birds in Europe
C. New Findings                 D. Migrating in V formation