The knowledge that some birds migrate great distances is relatively recent,just more than a hundred years old, in fact. There were some clues before that — in 1822 a German nobleman noticed a white stork on his estate with a strange object sticking out of it, so he shot it and the object turned out to be an arrow from Central Africa. This was regarded as mysterious and inexplicable, nobody seriously believed the bird had flown from Africa to Germany.
Ornithologists in northern Europe started trapping birds and putting identification rings on their legs in the early years of the 20th century. In 1911 a swallow ringed in England was recovered in Natal in South Africa. Even this evidence was regarded as dubious. When a report was published in the journal British Birds an editorial note stated: “… it seems unreasonable to suppose that swallows proceed southwards down the east coast of Africa, as might be inferred from this record.”
But soon after that the startling fact that many birds, including small ones like swallows and warblers, travel huge distances twice a year was fully accepted. The next question was how do they navigate?
It has yet to be fully answered, despite a huge amount of research. Initially the best theory seemed to be that they used astral navigation — plotting their route by the sun, moon and stars, just as humans did before the invention of the compass and modern navigational aids. The most recent research indicates that astral navigation does play a part in the migration of some birds, particularly those that travel at night, but it’s a relatively minor component in a complex mix of direction-finding abilities.
Nowadays there are believed to be two major components — their ability to sense the earth’s magnetic field and their ability to perceive polarised light. The physics involved in magnetic fields and polarised light are quite complicated and exactly how birds can sense things that we can’t is still open to debate. There is a detailed and up-to-date summary of our knowledge of the subject in Anthony McGeehan’s book, Birds of the Homeplace.
One fascinating fact I learnt from this book is that the Vikings may also have used polarised light. They used a ‘sun-stone’ which appears to have been a piece of rock crystal which utilised polarised light to glow with a blue colour when it was pointing towards the sun, even on completely overcast days.
In evolutionary terms birds have been around between 80 and 100 times longer than modern humans. It seems that they have learnt a few very clever tricks over that immense span of time.