IELTS Academic Reading passage: The Navigation of Migratory Birds

The Navigation of Migratory Birds

Each autumn, billions of birds abandon the breeding grounds of the northern hemisphere and travel towards warmer latitudes, returning the following spring to raise their young where they were themselves born. The distances involved are difficult to grasp. The Arctic tern, the most extreme traveller among them, moves between the polar regions twice a year and may cover forty thousand kilometres in a single annual cycle. What continues to puzzle biologists is not the endurance these journeys demand but the accuracy with which they are performed. Many species migrate at night, alone, across oceans that offer no landmarks of any kind, and a substantial proportion of them are young birds making the journey for the first time, without the guidance of an experienced adult.

Part of the answer lies in the planet itself. Birds possess a magnetic compass, and two mechanisms have been proposed to explain it. The first involves cryptochromes, light-sensitive proteins in the retina whose chemical reactions appear to be modulated by magnetic fields, which would mean that a migrating bird effectively sees the field superimposed on its visual surroundings. The second involves crystals of magnetite, a naturally magnetic mineral, found in tissue around the upper beak of several species. Laboratory work has established one important detail: the avian compass does not respond to polarity in the way a walker's needle does. Instead it reads the angle at which the field lines meet the surface of the earth, an inclination compass that separates poleward from equatorward but cannot, on its own, tell north from south.

The sky supplies a second set of cues. Birds migrating by day take bearings from the sun, compensating for its movement by means of an internal clock, so that a bird kept under artificial daylight shifted by six hours will depart on a course predictably rotated from the correct one. Night migrants read the stars, but not, as was once assumed, by recognising individual constellations. Experiments conducted in planetariums during the 1960s demonstrated that young birds learn the axis around which the night sky appears to turn, and that this rotational centre, rather than any particular star, defines north for them. Nestlings raised beneath a sky turning about a false centre adopted that false centre as north, and oriented accordingly for the rest of their lives.

Closer to the ground, migrants also use what they can see and, more surprisingly, what they can smell. Coastlines, mountain ridges and river valleys are followed as leading lines, and experienced individuals return year after year to the same stopover woodlands, sometimes to the same hedgerow. Homing pigeons displaced from their lofts appear to consult an olfactory map, associating particular atmospheric odours carried on prevailing winds with particular directions from home; birds whose sense of smell has been temporarily blocked are markedly worse at choosing a homeward bearing. Whether wild migrants rely on odour to the same degree remains contested, and the evidence outside the pigeon literature is thin.

None of this explains how a bird that has never migrated knows where to go. The prevailing account is that first-year birds inherit a simple programme, often described as clock-and-compass: fly in a given direction, for a given length of time, and stop. The programme is astonishingly specific. Warblers held captive during the migration season become restless at night and orient in the direction their free-living relatives are taking, and the number of nights over which this restlessness persists corresponds closely to the duration of the real journey. Crossbreeding populations with different routes produces offspring that head off on an intermediate bearing, which is difficult to explain unless the direction is written into the genome.

The limits of the inherited programme were exposed by a classic experiment. In 1958 the Dutch ornithologist A. C. Perdeck captured thousands of starlings migrating through the Netherlands and released them several hundred kilometres to the south, in Switzerland. The adults, which had made the journey before, corrected for the displacement and reached their usual wintering areas. The juveniles did not. They continued on the heading they had inherited and spent the winter in an area parallel to, but far from, the one their species normally occupies. The conclusion drawn from this, and confirmed many times since, is that the compass is innate while the map is learned, and that a young bird acquires the ability to determine its position only through the experience of having travelled.

These ancient systems are now being disrupted at speed. Artificial light draws nocturnal migrants off course and holds them circling above illuminated structures until they are exhausted, and several cities have found that darkening tall buildings on peak migration nights reduces collisions sharply. Warming springs advance the emergence of the insects on which many migrants depend, while the birds themselves, cued by day length rather than temperature, arrive on an unchanged schedule and increasingly miss the peak. The chain is only as strong as its weakest link: a species may be abundant on its breeding grounds and secure in its wintering range, and still decline steeply because a single stopover marsh, where it must double its body weight in a fortnight, has been drained.