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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 birdnavigation 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 intheir 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