【易伯华出品】雅思阅读机经真题解析--Bird migration

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A

Birds have many unique design features that enable them to perform such

amazing feats of endurance. They are equipped with lightweight, hollow bones,

intricately designed feathers providing both lift and thrust for rapid flight,

navigation systems superior to any that man has developed, and an ingenious heat

conserving design that, among other things, concentrates all blood circulation

beneath layers of warm, waterproof plumage, leaving them fit to face life in the

harshest of climates. Their respiratory systems have to perform efficiently

during sustained flights at altitude, so they have a system of extracting oxygen

from their lungs that far exceeds that of any other animal. During the later

stages of the summer breeding season, when food is plentiful their bodies are

able to accumulate considerable layers of fat, in order to provide sufficient

energy for their long migratory flights.

B

The fundamental reason that birds migrate is to find adequate food during the

winter months when it is in short supply. This particularly applies to birds

that breed in the temperate and Arctic regions of the Northern hemisphere, where

food is abundant during the short growing season. Many species can tolerate cold

temperatures if food is plentiful, but when food is not available they must

migrate. However, intriguing questions remain.

C

One puzzling fact is that many birds journey much further than would be

necessary just to find food and good weather. Nobody knows, for instance, why

British swallows, which could presumably survive equally well if they spent the

winter in equatorial Africa, instead fly several thousands of miles further to

their preferred winter home in South Africa Cape Province. Another mystery

involves the huge migrations performed by arctic terns and mud flat-feeding

shorebirds that breed close to Polar Regions. In general, the further north a

migrant species breeds, the farther south it spends the winter. For arctic terns

this necessitates an annual round trip of 25,000 miles. Yet, en route to their

final destination in far-flung southern latitudes, all these individuals overfly

other areas of seemingly suitable habitat spanning two hemispheres. While we may

not fully understand birds' reasons for going to particular places, we can

marvel at their feats.

D

One of the greatest mysteries is how young birds know how to find the

traditional wintering areas without parental guidance. Very few adults migrate

with juveniles in tow, and youngsters may even have little or no inkling of

their parents' appearance. A familiar example is that of the cuckoo, which lays

its eggs in another species nest and never encounters its young again. It is

mind boggling to consider that, once raised by its host species, the young

cuckoo makes its own way to ancestral wintering grounds in the tropics before

returning single-handed to northern Europe the next season to seek out a mate

among its own kind. The obvious implication is that it inherits from its parents

an inbuilt route map and direction-finding capability, as well as a mental image

of what another cuckoo looks like. Yet nobody has the slightest idea as to how

this is possible.

E

Mounting evidence has confirmed that birds use the positions of the sun and

stars to obtain compass directions. They seem also to be able to detect the

earth's magnetic field, probably due to having minute crystals of magnetite in

the region of their brains. However, true navigation also requires an awareness

of position and time, especially when lost. Experiments have shown that after

being taken thousands of miles over an unfamiliar landmass, birds are still

capable of returning rapidly to nest sites. Such phenomenal powers arc the

product of computing a number of sophisticated cues, including an inborn map of

the night sky and the pull of the earth's magnetic field. How the birds use

their 'instruments’ remains unknown, but one thing is clear: they see the world

with a superior sensory perception to ours. Most small birds migrate at night

and take their direction from the position of the setting sun. However, as well

as seeing the sun go down, they also seem to sec the plane of polarized light

caused by it, which calibrates their compass. Traveling at night provides other

benefits. Daytime predators are avoided and the danger of dehydration due to

flying for long periods in warm, sunlit skies is reduced. Furthermore, at night

the air is generally cool and less turbulent and so conducive to sustained,

stable flight.

F

Nevertheless, all journeys involve considerable risk, and part of the skill

in arriving safely is setting off at the right time. This means accurate weather

forecasting, and utilizing favorable winds. Birds are adept at both, and, in

laboratory tests, some have been shown to detect the minute difference in

barometric pressure between the floor and ceiling of a room. Often birds react

to weather changes before there is any visible sign of them. Lapwings, which

feed on grassland, flee west from the Netherlands to the British Isles, France

and Spain at the onset of a cold snap. When the ground surface freezes the birds

could starve. Yet they return to Holland ahead of a thaw, their arrival linked

to a pressure change presaging an improvement in the weather.

G

In one instance a Welsh Manx shearwater carried to America and released was

back in its burrow on Skokholm Island, off the Pembrokeshire coast, one clay

before a letter announcing its release! Conversely, each autumn a small number

of North American birds arc blown across the Atlantic by fast-moving westerly

tail winds. Not only do they arrive safely in Europe, but, based on ringing

evidence, some make it back to North America the following spring, after

probably spending the winter with European migrants in sunny African climes.

Questions 14-20

Reading passage 2 has seven paragraphs, A-G

Choose the correct heading for each paragraph from the list of headings

below. Write

the correct number, i-x, in boxes 14-20 on your answer sheet.

List of headings

i. The best moment to migrate

ii. The unexplained rejection of closer feeding ground

iii. The influence of weather on the migration route

iv. Physical characteristics that allow birds to migrate

v. The main reason why birds migrate

vi. The best wintering grounds for birds

vii. Research findings on how birds migrate

viii. Successful migration despite trouble of wind

ix. Contrast between long-distance migration and short-distance migration

x. Mysterious migration despite lack of teaching

14 Paragraph A

15 Paragraph B

16 Paragraph C

17 Paragraph D

18 Paragraph E

19 Paragraph F

20 Paragraph G

Questions 21-22

Choose TWO letters, A-E.

Write the correct letters in boxes 21 and 22 on your answer sheet.

Which TWO of the following statements are true of bird migration?

A Birds often fly further than they need to.

B Birds traveling in family groups are safe.

C Birds flying at night need less water.

D Birds have much sharper eye-sight than humans.

E Only shorebirds are resistant to strong winds.

Questions 23-26

Complete the sentences below using NO MORE THAN TWO WORDS from the

passage.

Write your answers in boxes 23-26 on your answer sheet.

23 It is a great mystery that young birds like cuckoos can find their

wintering grounds without ________.

24 Evidence shows birds can tell directions like a ________by observing the

sun and the stars.

25 One advantage for birds flying at night is that they can avoid contact

with ______.

26 Laboratory tests show that birds can detect weather without

________signs.

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