IELTS Reading Practice Test 2: Free, with Answers

AEvery autumn, billions of birds leave their breeding grounds in the northern hemisphere and fly south for the winter, some of them travelling more than 10,000 kilometres. Many make the journey alone and at night, and young birds often complete their first migration without any older bird to follow. Some return each spring not just to the same region but to the same garden or even the same nest. How they achieve such precision is one of the oldest questions in biology, and despite considerable progress, a complete explanation remains elusive.

BIt is now clear that birds do not rely on a single mechanism but combine several different 'compasses', depending on whichever is most reliable at a given moment. The first of these to be demonstrated was the sun. In the 1950s, the German biologist Gustav Kramer noticed that caged starlings became restless during the migration season and tended to face the direction in which they would normally fly. When he used mirrors to alter the apparent position of the sun, the birds' orientation shifted accordingly. Kramer also showed that the birds compensated for the sun's movement across the sky during the day, implying that they possess an accurate internal clock.

CNocturnal migrants cannot use the sun, and many of them rely on the stars instead. In a series of experiments in the 1960s, the American scientist Stephen Emlen placed indigo buntings in a planetarium, where he could control the pattern of stars on the ceiling. The birds oriented correctly under the artificial sky, but when Emlen rotated it, they changed direction to match. Surprisingly, he found that the birds had no inborn knowledge of particular constellations. Instead, young birds observed the rotation of the night sky and learned to identify its centre, which in the northern hemisphere lies close to the Pole Star. Birds raised under an artificial sky that turned around a different star later treated that star as north.

DThe most mysterious ability of all is the detection of the Earth's magnetic field. In the 1960s, Wolfgang Wiltschko, working in Frankfurt, showed that European robins kept in a closed room, with no view of the sun or stars, still tried to fly in the correct seasonal direction, and that altering the magnetic field around them changed the direction they chose. Subsequent work by Wiltschko and his wife, Roswitha, revealed something unexpected: unlike a human compass, the robins' compass did not register which end of the field pointed north. Instead, it responded to the angle at which the field lines meet the Earth's surface, which tells a bird whether it is facing towards the pole or towards the equator.

EThe physiological basis of this sense is still debated. For some years, attention focused on tiny particles of iron in the beaks of pigeons, which were thought to act as magnetic sensors. However, a study published in 2012 found that these particles were in fact inside cells of the immune system and were very unlikely to play any part in navigation. Most researchers now favour an explanation based on a protein called cryptochrome, which is found in the eye. When light strikes this protein, it may trigger a chemical reaction sensitive to the direction of the magnetic field, so that the bird could, in effect, perceive the field as a pattern superimposed on its normal vision. If so, the magnetic compass should function only in the presence of light, and experiments suggest that this is indeed the case.

FA compass, however, is not enough. To reach a particular destination, a bird must also know where it is in relation to its goal: it needs a 'map' as well as a compass. Evidence for this comes from a famous experiment carried out in the 1950s by the Dutch researcher Albert Perdeck. Thousands of starlings caught in the Netherlands during their autumn migration were taken by plane to Switzerland and released. The adult birds changed course and headed for their usual winter grounds in northern France and southern England. Juveniles on their first migration, however, maintained their original heading, and many ended up in Spain. The adults, it seemed, had learned a map from experience, while the young birds had only an inborn sense of direction.

GWhat this map consists of remains uncertain. Some scientists believe that birds use small variations in the strength of the magnetic field, which change gradually across the Earth's surface. Others have pointed to smell: in the 1970s, Floriano Papi and his colleagues in Italy found that homing pigeons deprived of their sense of smell frequently failed to return home from unfamiliar locations. A third, more speculative idea is that birds can hear extremely low-frequency sounds produced by ocean waves and mountains, which can travel thousands of kilometres. Different species, or even the same bird at different stages of its journey, may rely on different signals, which would help to explain why the question has proved so difficult to resolve.

HUnderstanding these mechanisms is no longer simply a matter of scientific curiosity. In 2014, researchers in Oldenburg, Germany, found that the robins they were studying could not use their magnetic compass on the university campus, apparently because of weak electromagnetic 'noise' from electrical equipment in the city. When the wooden huts in which the birds were tested were lined with aluminium screens, their sense of direction returned. Artificial light at night is also known to attract and confuse migrating birds, drawing them into cities where many die in collisions with buildings. As cities grow, knowing exactly which signals birds depend on may become essential to protecting them.

In April 2025, an American biotechnology company called Colossal Biosciences announced that it had brought back the dire wolf, a large predator that disappeared from North America more than 10,000 years ago. The three white pups had in fact been born to ordinary dogs acting as surrogate mothers, and they were grey wolves whose DNA had been edited in twenty places to give them some of the features of their extinct relative, including a larger body and a pale coat. Critics quickly pointed out that a grey wolf with a handful of changes is not the same thing as a dire wolf, a species whose ancestors separated from those of the grey wolf millions of years ago. Yet the announcement showed that 'de-extinction', the recreation of lost species, has moved from science fiction into the laboratory, and that it is time to ask seriously whether it is desirable.

There are three main ways in which a lost species might be recreated. The first is cloning, which requires living cells from the extinct animal and is therefore possible only for species that have disappeared very recently. The second is back-breeding, in which living animals that carry some of the extinct animal's characteristics are selectively bred to bring those features out again; this method has been used in Europe in an attempt to recreate the aurochs, the wild ancestor of modern cattle. The third and most ambitious approach is genetic engineering, in which the genome* of a living species is edited to resemble that of its extinct relative. This is the method behind Colossal's best-known project: an elephant adapted to the cold, which the company describes as a woolly mammoth.

The only extinct animal ever to have been brought back, however briefly, was produced by cloning. The bucardo, a wild goat of the Pyrenees mountains, became extinct in 2000 when the last female was killed by a falling tree. Scientists had already taken cells from her ear and frozen them, and in 2003 a cloned kid was born to a surrogate mother. It died within minutes because of a defect in its lungs. The bucardo therefore holds the unusual distinction of being the only animal to have become extinct twice. The episode illustrates a problem that applies to every method: even a successful birth is only the beginning, since a single animal cannot form a population, and a population with almost no genetic variety is unlikely to survive for long.

The most ambitious argument in favour of de-extinction is ecological. The Russian scientist Sergey Zimov has argued for decades that large grazing animals once kept the grasslands of northern Siberia open, and that their disappearance allowed the land to be covered by moss and shrubs. If animals similar to mammoths could be returned to the Arctic, he suggests, they would trample the snow in winter, allowing the cold to reach the ground and helping to keep the permafrost* frozen. Others, such as Ben Novak, who leads an effort to revive the passenger pigeon, argue that the techniques developed for de-extinction can help endangered species that are still alive. In 2020, for example, a black-footed ferret was cloned from cells frozen in the 1980s, adding much-needed genetic variety to one of North America's rarest mammals.

Opponents are not convinced. The ecologist Paul Ehrlich has called de-extinction a fascinating but foolish idea, warning that the promise of bringing species back could weaken the case for protecting them in the first place: if extinction is no longer permanent, why spend money preventing it? This concern, known as 'moral hazard', is difficult to test, but a study led by Joseph Bennett of Carleton University in Canada tried to put figures on a related problem. Bennett and his colleagues estimated the cost of maintaining populations of revived species in the wild and concluded that, if the money came from existing conservation budgets, the result could be an overall loss of biodiversity, because the same funds could protect a larger number of living species.

There are also concerns about the animals themselves. Cloning and genetic engineering both involve many failed pregnancies, and surrogate mothers of a different species may face risks during birth. An animal produced in a laboratory may also lack behaviour that it would normally learn from its parents: a mammoth calf raised by an elephant would not learn how to be a mammoth, because there are no mammoths left to teach it. This raises the question of what exactly is being created. Beth Shapiro, an evolutionary biologist who later became Colossal's chief science officer, has argued that an exact copy of an extinct species is impossible, and that the real aim should be to restore lost ecological functions rather than to reproduce the past.

This last point seems to be the key to the whole debate. If de-extinction is understood as the production of perfect copies of lost animals, it is bound to disappoint, and the claims made for it will continue to provoke justified scepticism. If, on the other hand, it is seen as one more tool for conservation, a way of restoring damaged ecosystems or rescuing living species from genetic decline, then the questions become practical ones: whether a particular project offers better value than the alternatives, and whether the animals it produces can live healthy lives. Such questions have no single answer, and they should be decided case by case, by scientists and the public together, not by private companies alone.

What must be avoided is the impression that extinction can now be reversed. Even the most optimistic supporters accept that only a small number of species could ever be recreated, at enormous cost, and that most of the animals and plants disappearing today will leave no frozen cells behind. The most effective way to deal with extinction remains, as it always has been, to prevent it.