24. A. heads　　 　　　　　　　 B. feelings　　　 　　　　　　　 C. muscles　　 　　　　　　　 D. spirits

23. A. attack　 　　　　　　　 B. bombing　　 　　　　　　　 C. competition　　　　　　　 　　　 D. struggle

22. A. admiring　　　　　　 B. exciting　　　 　　　　　　　 C. inviting　　　 　　　　　　　 D. threatening

21. A. advertising　 　　　 B. commercial 　　　　　　　 C. printing　　　 　　　　　　　 D. security

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

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.

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

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

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

50. We can infer from the passage that in the hotel ______.

A. you can read many newspapers from around the world

B. you can enjoy both the urban and rural exciting lives

C. there are many advanced computers for customers to use

D. there probably are some pictures of snow leopards

C

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.”