As the faint light begins to streak the hills behind us, the first fluting calls of the cranes can be heard. Slowly the birds seep into vision, first as a vague trembling mass, a shade lighter than the dark earth behind them. Then tall individuals stand out, starting to stride anxiously back and forth. With a manic gait straight out of Monty Python's Ministry of Funny Walks, they pace like geriatric generals pondering battle plans.

As the calling reaches a crescendo, five or six take off, flying low with deep flapping stokes. Soon big, twisting ropes of cranes are rising, forming knots and untying themselves again into elegant undulating lines as they head towards their feeding grounds. They rarely rise much higher than the two-storey tower from which we are watching them.

For perhaps ten minutes, there is a sense of miraculous abundance: no matter how many cranes fly off, the numbers on the ground seem undiminished. At last, however, the remaining cranes are getting visibly thinner on the ground. Within half an hour of the first flight, the salt pans are empty of these great birds.

Later that same day, driving past the same area, I caught sight of some huge birds very high above the lake. For a moment, I took them for vultures, so different was their wheeling flight from the lines of deliberately flapping cranes I had seen that morning. The leaders then began to fall from the sky, with an awkwardness no vulture ever displays. Wings spread like clumsy parachutes, legs stretched towards the earth, they fell in giddy, jerky circles, calling excitedly to each other.

As well they might. These birds had flown all the way from their wintering grounds in Extremadura, at the other end of Spain, and had recognised one of their favourite pit stops on their long journey to their breeding grounds in the north of Sweden.

Over the next few days, the numbers of the birds which had spent the winter in Gallocanta were swelled every afternoon by more arrivals from the south, spilling down from great heights, gradually becoming visible as though they had been spun from the sky itself.

Then, about a week later, a large section of the multitudes now assembled did something unusual. Thousands of cranes left their feeding grounds, and gathered in the fields right around the village, where I had never seen them previously. They were calling raucously. The manic walking started again, accompanied by hops and frantic flapping.

Five birds finally got into the air, then ten, then 20. Another group took off, and merged with the first, and then another, and then another, until there were great congregations flashing against the light, seeking and finding rising thermals. Visually, it was like their arrival from the south, but in reverse. As the birds rose higher they ressembled columns of fading smoke, or iron filings drawn relentlessly towards some magnet high above the Pyrenees. The next phase of their journey had begun.

A couple of weeks earlier, I had driven to Extremadura, close to the Portuguese border, in search of one of the cranes' favourite wintering grounds. One of Europe's bird heavens, the region draws tens of thousands of cranes down from northern Europe each year. The big attraction is the dehesa, an environment not unlike east African savannah, dominated by cork oaks scattered at intervals through pasture land of exceptional biodiversity.

The acorns that fall from these trees offer rich nutrition to the birds, and also to the famous local pigs. The latter end up dismembered, their haunches hanging in their thousands from the ceilings of smoky Spanish bars. There they are finely sliced on the counter to make melt-in your-mouth slices of jamón ibérico, one of Spain's signature gastronomic pleasures.

The birds are mostly luckier, fattening themselves up until the balmy Extremaduran winter turns up the heat towards spring in February, and they begin to gather in long flocks to start the long trip to the north. Local farmers are concerned that they compete with the pigs for acorns, but biologist Jesús Valiente is confident that the evidence shows that there is more than enough to go around.

Sure enough, some of the nuts at each tree we look at are distinctively pierced by the crane's long beak, others are chewed up and spat out by the pigs, and, though the birds are about to leave, still more still lie undisturbed on the ground.

These trees, of course, are valuable for another reason. Valiente is also upbeat, though other environmentalists disagree, about a crisis facing cork production in the region. Cork is a key economic argument for the preservation of the dehesa, but it is threatened by the growing use of plastic bottle-stoppers in the wine industry. He believes that growing demand in the US for cork for basketball courts, of all things, will shore up the industry. On such quirks of human consumption may the survival of a rare ecosystem and its emblematic birds depend.

Having got ahead of the cranes to reach Gallocanta, I then followed them across the Pyrenees to Les Landes, which in recent years has also become a major wintering area for them. As the western European crane population has soared, from 50,000 in 1971 to 150,000 in 2001, Iberia seems to have reached its quota, and the surplus birds are spending Christmas further north.

This may, of course, also be a product of global warming. But all such explanations are open to debate: we know that hundreds of years ago cranes did not all migrate north in winter, but remained in Spain and southern France to breed. The breeding range is spreading south again through Germany to northern France.

For the first time in 400 years, cranes are again nesting in Britain. And, as population pressure makes the western European crane flyway bulge, birds are wandering as far west as Ireland, where, according to Gordon D'Arcy's Lost Birds of Ireland, it was hunted to extinction by the late Middle Ages.

One crane turned up at Ashton's Callow, Co Tipperary, last April. D'Arcy believes the crane may return soon to the bog habitat currently being abandoned by Bord na Móna. Incidentally, the bird often called the "crane" in Ireland is in fact the smaller and unrelated grey heron, and this common misnomer may echo a lost historical memory.

The cranes that winter in Les Landes are, once again, the unlikely beneficiaries of human activity. At Arjuzanx, where I found wave after wave of cranes arriving from Spain in the evening, they roost in new wetlands produced by abandoned surface coal mines. Nearby, at Captieux, they enjoy the very special protection inadvertently offered by a French military zone, which is off-limits to the public. This is oddly reminiscent of the "accidental paradise" described by the US novelist and naturalist Peter Mathiessen in his marvellous book, The Birds of Heaven, when he found that the heavily-mined DMZ between the two Koreas had become a haven for much rarer Asian crane species.

Finally, the cranes led me much further north, to the shores of Lake Hornborga in southern Sweden. Here, at Trandansen, the "crane-dancing place", the intersection between the human and avian worlds is most explicit. For the Swedes, the return of the cranes has always been deeply symbolic of a longed-for spring. Every year, up to 50,000 citizens turn out to watch, from behind a wire fence, an extraordinary, if rather artificial display.

In a picturesque location, between a country church and the lake, tons of corn are distributed to attract the incoming birds. As many as 10,000 gather at a time. Because the breeding season is upon them, often exhibit their marvellous mating ritual, complete with evocative unison calls between couples, which looks so like our own courtship dances.

Dwarfing even the magnificent whooper swans scattered amongst them, the cranes at Trandansen dramatically demonstrate the role of what naturalists call "charismatic megafauna". These are species, like the Panda in China and the bald eagle in the US, which appeal to people without any other interest in wildlife, and become symbols for conservation campaigns.

The Eurasian crane's remarkable success in Europe over the past half-century is a triumph of conservation, and one that now suffers some of the problems of success, as farmers complain increasingly of crop damage. The Eurasian crane's expansion is an inspiration for those trying to save species whose total numbers can only be counted in hundreds, like the whooping crane and the Siberian crane.

Of course, the cranes at Trandansen know nothing of this. They will soon head north yet again, to the wooded wetlands of northern Scandanavian, where they will disperse and attempt to breed, with a modicum of privacy from their own kind, and from ours.

Paddy Woodworth is researching a book on habitat restoration and species reintroduction

ONLINE:

www.grus-grus.com (this will provide links to places mentioned in the article)

US sites:
www.savingcranes.org
www.operationmigration.org

Illustration above: Killian Mullarney