The invisible thread that pulls migratory birds from one pole to another remains a mystery, writes Paddy Woodworth.
The migration of birds is almost as deeply rooted in our sense of the changing seasons as the falling of leaves or the budding of new green growth. You don't have to be an ornithologist to feel the year slipping away when the swallows start gathering on telephone wires, or experience a little lift in your heart when the first one flits across your field of vision the following spring.
The biannual shift in bird populations caught the eye of biblical prophets such as Jeremiah: "The stork in the heaven knoweth her appointed times; and the turtle \ and the crane and the swallow observe the time of their coming". The phenomenon fascinated Aristotle, who got the migration of cranes and pelicans more or less right, but insisted that swallows denuded themselves of their feathers and hibernated in muddy riverbanks. Such was his authority that the eminent naturalist Carl Linnaeus still accepted this theory in the 18th century.
A colleague who knows about these things tells me that the mystery of bird migration was one of the favourite topics of hippies in London squats in the 1960s. After the third or fourth joint, he says, someone would inevitably raise the question with almost religious awe: "And those birds, man, how do you think they know how to get down to Africa and back?" The fact is that even the most clear-headed scientists today are not entirely sure how birds navigate, although they advance a number of overlapping theories.
It is generally accepted that there is a genetic factor, although controlled experiments have shown that migration routes can be significantly altered within three generations, a remarkable evolutionary fast track.
It is evident that some birds simply follow where their parents lead, as is the case with ducks and geese. Most other birds, however, travel separately from their parents, but arrive in more or less the same places.
The cuckoo, of course, never encounters its biological parents, which head south while it is still being fed by an unwitting foster parent. But the young cuckoo knows exactly what route to take, following no cue from whatever unfortunate bird has nurtured it in error. There could be no stronger case for arguing that detailed instructions for when and where to travel, as well as the urge to migrate, come wrapped up with the cuckoo's genes.
Aids to navigation include the sun, moon and stars, major landmarks (the island of Sicily, a pass in the Pyrennees) and even the earth's magnetic fields. Scott Weidensaul, author of the authoritative and beautifully written Living on the Wind, maintains "birds may use different suites of navigational clues, falling back on one as another fails".
Birds migrate primarily because seasonal change makes food scarce in one latitude and abundant in another. But hunger and dropping temperatures are not the main stimuli to start their long and dangerous journeys, although they influence timing. The real trigger seems to be the declining hours of daylight.
These theoretical speculations, however, pale before the wonder of witnessing the spectacle itself. Weidensaul usually maintains scientific rigour despite his evident passion for birding. But he sometimes plunges into deliberate fantasy when he needs to describe his direct experience of witnessing major migration events.
Here is how he recalls seeing a massive "fall" of thousands of exhausted songbirds which had just crossed the Gulf of Mexico to Florida in early spring: "The truth about migration is that birds are conjured from the soft April air of a Gulf Coast sky. The blue is rolled up to make indigo buntings and cerulean warblers, the fog folds in on itself to birth gray catbirds and gnatcatchers, while the orange clouds at dusk give of themselves to create oriols. Once the sky is full to bursting with these new-made wonders, it lets them fall like snow on land. Poetic hogwash, you say? Suit yourself. I've seen it happen."
The price of this kind of magical rapture is usually several hours of bitter discomfort, cold and insect bites. Now, however, some of the magic can be captured in the comfort of a cinema, courtesy of Jacques Perrin, whose ground-breaking documentary will make you believe a man can fly with birds.
Winged Migration shows us birds as most of us have never seen them before, in flight at great height. It is as though we are able to move among them, following the visible curve of the planet from one continent to another. David Attenborough occasionally got effects such as these for a moment or two in The Life of Birds - there is a famous shot which lets us feel as though we are flapping along beside a drake mallard - but nothing as sustained as Perrin's breath-taking achievement.
Occasionally, it can appear that the technology betrays the reality. His cameras periodically capture a bird which seems to be suspended, moving its wings energetically but going nowhere, as though it were exercising in some sort of ethereal gym.
Then, of course, you realise that this is often precisely how one high-flying bird would see its fellow migrants; when there is no background except an infinite blue void, there is nothing against which to measure forward movement.
Perrin is not afraid of broad strokes - he brings us with barnacle geese past the skyline of New York; with greylag geese above the Eiffel Tower, with bar-headed geese along the Great Wall of China. But his best effects are much more subtle, and much more powerful.
In one very moving sequence, a small flock of red-breasted geese drops exhausted into a factory's overspill pools in a polluted wasteland in eastern Europe. At first they seem to find the warm, steaming water a blessed relief after their exertions, even as huge crane-trolleys trundle almost on top of them. As the geese emerge, apparently onto mud, one of them sinks and finds its flight pinions mired in oil. The others take flight immediately, leaving their companion to a grim, slow death.
Geese, you will have gathered, feature heavily in the movie. Perrin almost always goes for the big, spectacular migrants: cranes, storks, swans and other waterfowl.
Obviously these birds are easier to film, and more familiar to a general audience, but it seems a pity that no serious attempt is made to cover the massive migrations of songbirds. These can be, as Weidensaul describes so graphically, as spectacular in their own way as the "big game" of the bird world.
Even wading birds, such as redshanks and sandpipers, which perform such glittering kinetic ballets when flying in large flocks - as anyone familiar with Dublin's Bull Island will know well - only get a perfunctory look-in.
Perrin could also be accused of over-egging the cake - including footage which, although very rich in itself, impedes his rather meandering attempts to impose a storyline on his subject.
But these are minor quibbles in a work which provides unprecedented visual access to one of the great natural marvels. The film's underlying message, that this delicate network which extends across the planet is perilously frayed by ecological damage, is all the more effective for being understated.
And while Perrin patently feels passionate about wildlife, he is not sentimental about it. A devastating scene in which an injured tern is chomped up by an army of ferocious crabs is an unblinking vision of nature "red in tooth and claw".
To the layperson who has occasionally noticed a flock of geese in the October sky, as well as for the committed ornithologist, birds will never look quite the same after seeing this film.
Paddy Woodworth is researching a book about images of bird migration in folklore, literature and the arts
Winged Migration goes on release in cinemas on Friday, October 17th