geese, sometimes individual birds that have made their twice-annual trek more than fifty times. A ringed (banded) snow goose hatched in Alaska and wintering in Mexico has been recorded still migrating at twenty-six years old – that’s more than 130,000 miles of reading the wind. I ask myself just what huge range of conditions and changes, trials and close-calls lurk behind the twinkle in that wise old bird’s eyes.
We also know that different families of birds respond to and navigate by different signals, reflecting each species’ needs and capabilities and determining their route and destination. Experiments in planetaria have proved that some Silviid warblers, such as the blackcap, are genetically wired to navigate by the stars, requiring them to migrate at night. Artificially exposed to different seasonal constellations, caged birds become restless and flutter to the north or south, according to their migratory instincts. Other species can detect the Earth’s magnetic field or memorise significant landmarks, such as coastlines, river valleys and mountain ranges. Yet others follow the sun, demonstrated by German ornithologist Gustav Kramer’s 1950s experiments with caged starlings. Most significantly, he proved, with mirrors and artificial cloud effects, that it wasn’t direct sun they required, but that sufficient light intensity was all they needed for the correct orientation – an important ability for birds since the sun is so often obscured by clouds.
One of the most remarkable experiments in bird navigation was conducted by my late great friend Ronald Lockley, a real pioneer of ornithological research and author of the ground-breaking monograph Shearwater (1946) – a study of Manx shearwaters nesting on the island of Skokholm off the Pembrokeshire coast of Wales. He took mature birds from their nesting burrows, then shipped them off to Venice and to landlocked Basel in Switzerland, many miles from any normal shearwater habitat or migratory route. They were back in their burrows fourteen days later.
Another bird was flown across the Atlantic to Boston, Massachusetts, some three thousand miles from home; a starting point completely unknown to that shearwater species. It took just twelve days to arrive back on Skokholm. What Lockley’s experiments proved conclusively was that shearwaters must use navigational aids other than landmarks. What we now know is that birds often employ a combination of abilities: stellar, solar, geo-magnetic and geographical recognition, to locate themselves and return to precisely the same wood, moor, field, tree or bush, swamp, stream, burrow or cranny they departed from many months before. Our swallows swoop home from Africa to the rafters of their birth through the same door in the same stable at approximately the same moment, year after year. Nowadays we know so much that we take bird migration for granted, but it was not always so.
In the early sixteenth century the Bishop of Uppsala, Olaus Magnus, was convinced that swallows and other similar birds spent the winter months under water or in deep mud. In his Historia de Gentibus Septentrionalibus he goes so far as to cite the evidence of fishermen 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.
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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
