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11 July 2009
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Unravelling the secrets of bird migration.
Birds are highly mobile and among the swiftest living creatures. Flight gives them the power to move in any direction for as long as they have the energy to keep going. They are equipped with lightweight, hollow bones, navigation systems that leave NASA in the shade, 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 evolved a system of extracting air from their lungs that far exceeds any other animal. In each natural region of the world, each country, each county and even each parish, birds come and go with the seasons. Many take on arduous journeys to reach us and although a species like the swallow may breed and spend the summer in one place, it spends most of the rest of the year travelling back and forth to its wintering grounds in Southern Africa. On the way it faces constant danger and a daily requirement to find food, water and a safe overnight roost. So why do swallows and millions of other birds migrate, rather than stay in one place?
Most of the explanation revolves around food. In temperate and Arctic regions of the Northern Hemisphere food is abundant during a short growing season. Migration evolved as a way for birds to exploit resources that are seasonally abundant and, equally important, to go elsewhere when the resources become scarce or harsh weather arrives. However, many species can tolerate cold temperatures if food is plentiful; when food is not available they must migrate. While this simple explanation for why birds migrate is essentially true, intriguing questions remain. One puzzling fact is that many birds journey much further than would be necessary to find food and good weather. Presumably, British swallows could survive equally well if they spent the winter in equatorial Africa instead of flying, as they do, several thousands of extra miles to their preferred winter home in South Africa’s Cape Province.
Arctic tern Another apparent conundrum involves the huge migrations performed by arctic terns and mudflat-probing shorebirds that breed close to polar regions. In general, the further north a migrant species breeds, the further south it spends the winter. For arctic terns this necessitates an annual round trip of 25,000 miles. Yet, en route to a final destination in far-flung southern latitudes, all these individuals overfly areas of seemingly suitable habitat spanning two hemispheres. Such compulsive journeys probably evolved over aeons of time and in response to fluctuations in weather, especially the distribution and movement of ice sheets. While we may not fully understand birds’ reasons for going to particular places, we can marvel at their globetrotting feats.
Just how do they find their way and, more to the point, how do young birds migrate over vast distances without parental guidance? Very few adults migrate with juveniles in tow and youngsters may have little or no inkling of their parents’ appearance. Cuckoos are famous for depositing their eggs in the nests of other birds that unwittingly serve as foster-parents. Astonishing as this piece of evolutionary behaviour is, it is even more mind boggling to consider that, once raised, the young cuckoo makes it own way to ancestral wintering grounds in the tropics before returning single-handedly to northern Europe to seek out a mate among its own kind.
As mankind’s research has dug deeper into the fine print of bird migration, its mystery has been both better understood but also compounded. Mounting evidence has confirmed that birds use sun and stars to obtain compass directions but true navigation 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. In one instance a Welsh manx shearwater transported and released in America was back in its burrow on Skokholm Island off the Pembrokeshire coast one day before a letter announcing its release!
Manx shearwater Such phenomenal powers are the product of computing a number of sophisticated cues including an innate 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 than we do. 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 see the plane of polarized light caused by it. This 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 structure is cool and smooth and conducive to sustained, stable flight.
Sedge warbler 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 favourable winds. Birds are adept at both and, in tests, some can 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, that 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.
Dowitcher Unravelling the secrets of bird migration is as absorbing as a good detective story. Sadly, some never complete their journeys. Not so long ago the assumption was that a bird in the air on a dark windy night with no visible points of reference to judge position was, like a pilot without radar, hopelessly lost. We now know this is not the case. Losses do occur, but mainly because birds get swept off course by strong air masses against which battle is futile, resulting in a watery grave. Indeed, it is not unusual for ocean fish to be caught with bird remains in their stomachs. But sometimes there’s a silver lining. Each autumn a small number of North American birds are 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 in the following spring - after probably spending the winter with European migrants in sunny African climes. Well, if you had wings, who wouldn’t?
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