资料题：We could see that, at that time, the boy was very sad since（　）。

As skies are filed with millions of migrating birds, European scientists say the seasonal wonder appears to be strange; the fatter that bird, the better it flies.The results of their study led to a theory opposite to a central one of aerodynamics, which says that.the power needed to fly increases with weight.For birds, obviously, the cost of flying with heavy fat is much smaller than we used to think.Researchers found that red knot wading birds double their normal:body weight of 100 grams before making their twice.- a- year nonstop flight between the British Isles and the Russian Arctic.Distance: 5,000 kilometers.Another study in the magazine Nature measured the advantage of flying in an aerodynamic group which allows birds.to save energy by flying smoothly and quietly in the lead bird's air stream.Flying in groups, their heart rates were 14.5 percent lower than flying alone, according to Henri,a French scientist.The findings help explain how birds complete difficult migrations.Researchers.had thought that thinner, stronger birds would have the best chance to survive.The first study suggests that building up fat to be burnt as fuel during the migration 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 weights during 28 simulated fights.They forced a small amount of special water into the birds' bodies so that they could measure the amount of energy burnt during the fight.