It is known to all that technological and scientific advances have made more remarkable changes to the range and quality of our food. Some people regard it as an improvement, while others believe that the difference is harmful. Discuss both views and give your own opinion.

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 performed efficiently during sustained flights at altitude, so they have a method 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 accumulate multiple layers of fat to provide sufficient energy for their long migratory flights. B The fundamental reason

birds

migrate is to find adequate

food

during the

winter

months when it is in short supply. It 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 unavailable, they must migrate;

however

, intriguing questions remain. C

One

puzzling fact is that many

birds

journey much

further

than necessary 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

of flying several thousands of miles

further

to their preferred

winter

home in South Africa cape province. Another mystery involves the massive migrations performed by arctic terns and mudflat-feeding shorebirds that breed close to polar regions in general; the

further

north a migrant

species

Breeds, the

further

south it spends the

winter

. For artic terns,

this

necessitates an annual round trip of 25000 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

find traditional wintering areas without parental guidance. Very few adults migrate with juveniles in tow, and youngers may even have little or no clue about their parents’ appearance. A familiar example is a cuckoo, which lays its eggs in another

species

nest and never reencounters its young. It is mind-boggling to consider that, once raised by its host

species

, the young cuckoo makes its way to ancestral wintering grounds in the tropics before returning single-handedly to northern Europe the

next

season to seek out a mate among its 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

also

seem to detect the earth’s magnetic field, probably due to having minute crystals of magnetite in the region of the brain.

However

, accurate 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

can still return rapidly to nest sites.

Such

phenomenal power is the product of computing several sophisticated cues, including a genetic map of the

night

sky and the pull of the earth’s magnetic fields. 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

, seeing the sun go down, they

also

seem to know the plane of polarized light caused by it, which calibrates their compass. Travelling 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 calm and less turbulent and conducive to sustained, stable flight. F

Nevertheless

, all journeys involve considerable risk. And part of the skill in arriving

safety

Replace the word

safely

show examples

is setting off at the right time.

This

means accurate

weather

forecasting and utilizing favourable winds.

Birds

are adept at both, and, in the laboratory test, 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. At the onset of a cold snap, Lapwings, which feed on grassland, flee west from the Netherlands to the British isles, France and Spain. 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 American and released was back in its burrow on Stockholm island, off the Pembrokeshire coast,

one

day before a letter announcing its release!

Conversely

, each autumn, a small number of North American

birds

are blown across the Atlantic by fast-moving westerly tailwinds. 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.