英语阅读题It’s food scarcity (短缺),not dietary preferences,that mo

英语阅读题
It’s food scarcity (短缺),not dietary preferences,that motivates birds to migrate thousands of miles back and forth between breeding and non-breeding areas each year,new research shows.
“It’s not whether you eat insects,fruit,nectar or candy bars or where you eat them — it matters how reliable that food source is,” said study leader W.Alice Boyle of the University of Arizona.
To figure out the pressures that drive some birds to leave home for the season,Boyle examined 379 related species of flycatchers and compared their sizes,food types,habitats,migratory behaviors and whether or not they fed in flocks.
To compare the birds,the researchers constructed a “supertree” showing the exact evolutionary （演化的） relationships among different species.
A computer analysis then determined whether a particular species was migratory because it ran in the “family” or whether something in the bird’s environment was forcing it to leave each season.
Boyle and her colleagues found that food scarcity was the number one issue that predicted a species’ migratory behavior.
An alternative strategy that birds use to deal with food scarcity is to forage (四处搜寻) in flocks,because a group is more likely to find a new source of food than a lone individual.
“If you are faced with food scarcity,you have two options,” Boyle said.“You can either forage with other birds,or you can migrate.”
51.The passage mainly tells us ________.
A.the real reason for birds’ migration B.food scarcity that birds face today
C.how birds search for food D.when birds migrate
52.The scientists used to believe that birds migrate because ________.
A.they want to search for the food they like
B.they can find more food in a different place
C.they can breed their young in a different place
D.the weather turns too cold
53.The researchers constructed a “supertree” mainly to ________.
A.compare the sizes of different birds
B.make out the relationships between different birds
C.find out what causes birds to leave each season
D.predict when the birds were migratory
54.Which of the following is true?
A.All birds fly to non-breeding areas each year.B.There are 379 species of birds migrating.
C.Two related species have the same behavior.D.Computers are used in the research work.
55.The passage suggests that ________.
A.different kinds of birds have different ways to migrate
B.different kinds of birds migrate for various reasons
C.some birds can survive without migrating each season
D.birds’ food sources are becoming rare all over the world
多用中文,再粗略给我讲讲这篇在说什么

51. A
依据：It's food scarcity (短缺), not dietary preferences, that motivates birds to migrate thousands of miles back and forth between breeding and non-breeding areas each year, new research shows.
52. A
依据：It’s not whether you eat insects, fruit, nectar or candy bars or where you eat them — it matters how reliable that food source is.
53. B
依据：To compare the birds, the researchers constructed a “supertree” showing the exact evolutionary （演化的） relationships among different species.
54. D
依据：A computer analysis then determined whether a particular species was migratory because it ran in the “family” or whether something in the bird’s environment was forcing it to leave each season.
55. C
依据：An alternative strategy that birds use to deal with food scarcity is to forage (四处搜寻) in flocks, because a group is more likely to find a new source of food than a lone individual.
If you are faced with food scarcity, you have two options,” Boyle said. “You can either forage with other birds, or you can migrate.”
文章大意：
现代研究表明决定鸟儿迁移的主要原因是食物的短缺而不是饮食上的偏好.科学家使用了“超级树”模型和电脑演算出这一结论.
鸟儿应付食物短缺一般有两种方式,一是迁移远方,二是结伴搜寻食物.