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landing a catch of swallows in their nets. He was sufficiently persuaded by this bizarre explanation for the sudden disappearance of swifts and swallows in the winter that he commissioned a woodcut illustration of the fishermen with their catch, thereby endorsing an entirely bogus scientific claim that would remain substantially unchallenged for the best part of a hundred and fifty years.

      There were doubters and fence-sitters, of course, casting around for objectivity, such as Robert Burton, who wrote in his 1621 Anatomy of Melancholy:

      Do they sleep in winter . . . or lye hid in the bottom of lakes and rivers, spiritum continentes? so often found by fishermen in Poland and Scandia, two together, mouth to mouth, wing to wing, and when the spring comes they revive again . . . Or do they follow the Sun . . . or lye they in caves, rocks and hollow trees, as most think . . .?

      Yet this thesis sat uncomfortably with the drip-drip of evidence coming in from ships returning to British ports from the Mediterranean and Africa with tales of exhausted swallows landing on rigging or on decks.

      John Rae, the great English naturalist of the seventeenth century, editing Willughby’s Ornithologica in 1678, certainly expressed doubt: ‘To us it seems more probable that they fly away into hot countries, viz., Egypt or Aethiopia.’ But others would have none of it. Even the great Swedish taxonomist Carl Linné (Linnaeus) was insisting as late as 1768 that Bishop Olaus’s confident assertions, with all the authority of the Church, were correct – that they hibernated under water.

      By the end of the eighteenth century the ever more divided world of science had split clearly into migrationists and hibernationists. Gilbert White, curate of Selborne, was well aware of the debate. His lengthy correspondence with the Hon. Daines Barrington (hibernationist) and Judge Thomas Pennant (migrationist), both eminent naturalists of their day and Fellows of the Royal Society (and from which correspondence much of the text of his 1789 Natural History and Antiquities of Selborne was gleaned), reveals strong influences in both directions, but White was canny and stuck to a much more cautious scientific approach: ‘As to swallows being found in a torpid state in the winter in the Isle of Wight or any part of this country, I never heard any such account worth attending to.’ But with the scientific objectivity that would make his natural history so famous (never out of print in 225 years), he also hedged his bets.

      I myself on the 29th October last (1767) . . . saw four or five swallows hovering around and settling on the roof of the (Oxford) county-hospital. Now is it likely that these poor little birds . . . should, at that late season of the year . . . attempt a journey to Goree or Senegal, almost as far as the Equator? I entirely acquiesce with your opinion – that though most of the swallow kind may migrate, yet that some do stay behind and hide with us during the winter.

      Gilbert White’s brother was chaplain to the British garrison on Gibraltar. Also a keen naturalist and a reliable observer, John sent his brother reports of swallows crossing the strait to Africa. This first-hand evidence enabled Gilbert to write back to the devout hibernationist Daines Barrington:

      You are, I know, no friend to migration; and the well-attended accounts from the various parts of the country seem to justify you in your opinions, that at least many of the swallow kind do not leave us in winter, but lay themselves up like insects and bats in a torpid state . . . But then we must not, I think, deny migration in general; because migration does subsist in some places, as my brother in Andalusia has informed me. Of the motions of these birds he has ocular demonstration, for many weeks together, both spring and fall: during which periods myriads of the swallow kind traverse the Straits from north to south and from south to north according to the season.

      This may well have irritated Barrington, because shortly afterwards he published a damning paper utterly refuting the whole idea of bird migration. Although White was totally convinced that bird migrations were real, he was evidently puzzled by the late movements of some species, often well into November, but his admirable scientific objectivity would never permit him to reject altogether, without positive proof, the hibernation possibility. Right at the end of his life he was still instructing labourers to search for hibernating birds in winter. In April 1793, only three months before his death, he asked a neighbour to assist him in examining the thatch on an empty cottage in Selborne.

      * * *

      Things are very different, these days. We possess an astonishing log of scientific knowledge about migrations of all sorts. Thanks to ringing (banding) and radio tagging, birds are perhaps the most studied, but so are elephants and polar bears, herds of antelope, the great wildebeest migration from the Serengeti across the Mara River, and the carnivores and scavengers that follow them; deer, like the caribou migration of the Alaskan tundra and their attendant wolf packs; whales and seals of many different species migrating to breed or feed; eels and basking sharks and many other migratory fish; reptiles, such as turtles and crocodiles; and, of course, insects in uncountable numbers. Billions of monarch butterflies migrate up to 2800 miles down the North American continent from Alaska to Mexico because they can’t withstand the cold winters.

      Some of the research aided by modern technology has revealed previously undreamed-of feats of endurance and ability. Imagine their surprise when the pilots of an Air India passenger jet found themselves flying alongside a large skein of bar-headed geese at thirty-two thousand feet, an altitude required every year as the geese cross the highest Himalayan peaks.

      Radio transmitters have revolutionised bird research. Very recently, an electronic tracking device weighing less than a paperclip uncovered what is now thought to be one of the world’s greatest bird migrations. It revealed that a red-necked phalarope, a tiny wader the size of a wagtail, migrated thousands of miles west across the Atlantic to the Pacific Ocean, a journey never recorded for any other European breeding bird.

      Dave Okill, of the Shetland Ringing Group, fitted individual geo-locators to ten phalaropes nesting on the Shetland island of Fetlar. When a bird returned to Fetlar in the spring, Dave was astonished to discover that it had made an epic 16,000-mile round trip during its annual migration – across the Atlantic, south down the eastern seaboard of the US, across the Caribbean and Mexico, ending up off the coast of Peru, taking the same route back. Prior to this, many experts had assumed that Scottish breeding phalaropes joined the Scandinavian population at their wintering grounds, thought to be in the Arabian Sea.

      My conservation colleague Roy Dennis has hugely increased our knowledge of osprey, marsh harrier and golden eagle movements by attaching transmitters to young birds leaving the nest. The British Trust for Ornithology has done the same by attaching solar-powered radio tags to English cuckoos in an attempt to discover the route and precisely where our diminishing British cuckoo population spends the winter. The results have been illuminating and, for us, deeply disappointing. Aigas Field Centre sponsored one bird named Kasper; he made it to the Congo Basin for the winter, but perished on the way back in the spring – just our luck. Of the first five birds tagged in 2011, only two made it back to Britain.

      Sophisticated modern radar can also accurately track the movement of small passerine migrants. We now know that most small birds migrate at below five thousand feet, the most popular altitude being two to three thousand feet, whereas flocks of waders choose to travel much higher, at twenty thousand feet. We can also measure speed of flight very accurately. Warblers, finches and other small birds commonly cover thirty to fifty miles a night with daytime stopovers to rest and feed, whereas swifts, swallows and house and sand martins regularly cover up to two hundred miles a day, preferring to roost at night and fly by day so that they can feed on flying insects as they go.

      Raptors, such as ospreys and harriers, tend to move much more slowly, travelling by day and using thermals to spiral upwards so that they can glide for long distances before rising and repeating the process all over again. The exception to this rule may be falcons. Only twenty-four hours after it was ringed in Paris, a young peregrine falcon was gunned down on Malta, some thirteen hundred miles south, an average of fifty-four m.p.h. without stopping.

      * * *

      For me, here and now, migration means geese and swans, waders thronging the mudflats of the Firth and woodcock slinking into the woods. Our small summer migrants all vanished south long ago, but the onset of winter brings the Arctic species down to our more

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