Their calls bound them together in the air: 10, 20, 30 in a group, whole families together. They flew at motorway speeds and sometimes eerily high - almost as high as Everest - to catch the freezing draught of the jet stream: Iceland to Ireland in seven hours, across 1,000 kilometres of sea. Most flew much lower, perhaps for 40 hours in difficult winds, until the glittering sea loughs of Swilly and Foyle drew them down, black paddles skidding on the waves. Early arrivals stretched their necks towards them, trumpeting a greeting.

October's sudden influx of whooper swans at the northern loughs is one of dozens of migrations that make Ireland a hub and refuge for European and Arctic birds. As 11,000 whoopers fanned out to the island's lakes and rivers, they wove through the flights of other wildfowl arriving in Ireland for the winter: half-a-dozen sorts of geese from Canada and Greenland, assorted flocks of ducks from Iceland, Russia, and Scandinavia.

At Ireland's coasts, great flocks of wading birds from the Arctic - knot, dunlin, godwit, redshank - are probing for food in estuary mud they know will never freeze. For many of them, Ireland is a vital refuelling-stop on the way back to African coasts; others will stay for the winter.

Most of our island's summer migrants - swallows, warblers, wheatears and so on - have already gone south, but the autumn migration lacks the urgency of the spring surge north to breed. Swallows start moving in August, but a few stragglers may still be around two months later. Even the portly corncrake - such an unlikely migrant that folklore had it changing into a moorhen for the winter - can still be here in October.

As temperatures fall on the Continent, Scandinavian songbirds move south and west towards the Atlantic: flocks of redwings, fieldfares, starlings and chaffinches swirl above Ireland's berried hedgerows and soft grassland. Lapwings from Britain fly ahead of Continental snipe, curlew and woodcock. When Scandinavian rowan trees have lost all their fruit, colourful, crested waxwings can end up in Dublin and Belfast, feeding on the berries of suburban gardens.

Autumn regularly brings to Ireland a scatter of American birds whose own migrations down the western Atlantic coasts have been disrupted by gales, or even, as this year, by major hurricanes. Exotic waders and warblers make an exhausted landfall at the coasts of Cork and Kerry (some having taken rests on ships), there to be identified and added to the "life lists" of Irish birders excited by their rarity. In recent years, with more and more expert observers, these vagrants have added dozens of species to the roster of bird species seen in Ireland.

They have little to do, however, with the web of ocean migration that uses Ireland as a base for breeding, beside waters rich in spring plankton and teeming with fish. Our western islands support some of the Atlantic's most important breeding colonies of seabirds.

Some 27,000 pairs of gannets, for example, nest on Little Skellig off Kerry, turning the island white. At the Blaskets and other western islands, vast numbers of shearwaters and storm petrels breed in a honeycomb of burrows, coming to them only after dark to avoid attacks from great black-backed gulls. In autumn, they join the great southward flow of seabirds along the west coast: thousands every hour for days on end. Irish and British birders gather on headlands to spot rare petrels, shearwaters and terns in the distant passage of wings.

Seabirds hold the long-distance records for migration. Most Arctic terns, for example, spend their lives in annual commuting from one end of the planet to the other, breeding far north in the Arctic and wintering among the pack-ice of the Antarctic. In round journeys of perhaps 40,000 kilometres, they rarely see darkness. One of their flyways takes them past Ireland, where some 3,000 pairs drop out to nest in colonies on undisturbed islands off the west coast. Shearwaters, too, make immense ocean journeys, skimming the waves between Ireland and the Atlantic off Brazil. A Manx shearwater recovered at the Copeland Islands Bird Observatory off Co Down last summer had been ringed an astonishing 50 years ago.

But even small land birds have been drawn into long and often dangerous migrations. The little wheatear, with its white rump and chack-chack call, not only flies from west Africa to Ireland to breed: many press on from here to Iceland, Greenland and even eastern Canada, thus adding a whole ocean to the crossing of the Sahara. They arrive in the Arctic, very often, in a landscape still white with snow. Another race of the bird leaves east Africa and flies to Siberia and even Alaska for the few months of breeding.

Why do birds do it? Put crudely: because it pays. It offers marginal benefits that have been matched by no other behaviour in their history. By undertaking these extraordinary flights, with all their risks and casualties, more birds survive to rear more young to carry on their species than if they stayed in one place. The habit and skills of migration have thus been built into genes over thousands of years by the process of natural selection.

How did it start? It can be easy to think that birds "belong" in the places where they raise their young - that the Irish swallows now migrating south through Africa are leaving home to spend winter in the sun. But Africa, not Ireland, is where they began. The last ice age, which covered Ireland almost completely and pushed Arctic tundra conditions far to the south, is what explains the great length of many of today's migrations.

When the ice began to retreat, some 13,000 years ago, and vegetation, trees and insects moved northward again, birds began to move with them. The woodland birds were following their habitats, as residents. But other species, in crowded competition further south, were led to explore lands relatively empty of predators, with a wealth of insects in summer and long days in which to feed. Even in the High Arctic, the brief summer launches midges and mosquitoes by the billion, and sedges and heathers spring up from the tundra as food for wildfowl.

The journeys gradually grew longer, century after century. Birds began to leapfrog for greater advantage further north. As the pros and cons of travel were tallied in the genes of surviving birds, mere commuting turned into migration of whole species.

In looking at the thousands of migratory paths that criss-cross the globe, east-west as well as north-south, it would often seem to make more sense for some birds to take shorter routes, or to settle for different destinations at either end, but that supposes choice, which birds are rarely allowed. Some of Europe's east-west migrations in winter can seem more akin to "weather movements", in that falling temperatures can provide an obvious reason for leaving. But the return migration to breed needs the prompting of a body-clock. Most true migrations are made largely independently of local conditions, and with no way of knowing the weather at the other end.

They are triggered by changes in day-length and start with advance "feeding-up" to store fat (even in the eyelids) as fuel for the journey. Some warblers, little featherweights at any time, double their weight in readiness. Young swallows spend time learning the topography of the place where they were born before setting out on the journey already mapped in their genes.

This has been simply put as following an innate "time/distance programme" in which the birds obey a sequence of genetic settings, flying in a certain direction for a certain number of days, then in another for a further period, and so on. Most small birds fly at night to avoid predators. They spread out across a broad front and use star patterns to set their course (a series of Dutch experiments with starlings, using a planetarium, demonstrated this half a century ago). By day, the sun's position is both a guide to navigation and a measure of passing time.

But cloudy skies and bad weather can be ruinous to celestial navigation, and if migrating birds have a sixth sense in reserve it seems to be a perception of the Earth's magnetic field and its north-south orientation. Last month, scientists at the German University of Oldenburg described new research suggesting that birds can physically see magnetic fields, using specially responsive proteins in the retinal cells of the eye.

Swedish experiments also suggest that magnetic cues can warn small birds to replenish their body fat before crossing a desert or ocean. There could be other navigational aids beyond human experience. Perception of polarised light, for example, can locate the sun through cloud. Special sensitivity to particular sounds and smells may also be part of the birds' migratory repertoire.

Irish migration research, once pioneered through records kept at lighthouses, continues today with radio-tagging and elaborate ringing programmes. Films like the marvellous Winged Migration, the Oscar-winning documentary, take us wing-tip to wing-tip along with the birds as they cross continents, oceans and polar wilderness. The more we learn, the more we are left in awe at the intricate resorts of evolution.

Michael Viney writes the Another Life column for The Irish Times. His most recent book is Ireland: A Smithsonian Natural History (Blackstaff).