They have been the harbingers of spring and a promise of long summer days for people in the northern hemisphere for thousands of years. They are considered in all cultures from Asia, Europe, and North America to bring good fortune to those lucky enough to have them nesting near their homes.

References to the swallow's arrival have been found on inscriptions on ancient Greek vases, in the Bible and in the first century writings of the Roman naval commander and naturalist, Pliny. Even today, ask anyone to name a migrant bird and most will mention "the swallow".

Yet an understanding of its migration is a relatively recent development. Gilbert White, writing in The Natural History and Antiquities of Selbourn in 1788, concluded that swallows spend the winter hibernating in mud at the bottom of ponds and lakes, a theory shared by most eminent naturalists of the day and stemming from the fact that swallows were usually seen hawking insects over ponds and lakes before they disappeared for the winter. Then, in spring, the first birds would be seen again feeding over such wetlands.

And yet reality is almost equally unbelievable.

Barn swallows (the correct name for our swallows) begin to arrive in Ireland from late March on, with the main influx from mid-April. Their journey starts in early February as they begin to leave their wintering quarters in South Africa.

Feeding as they fly, they travel north over the Namib and Kalahari Deserts up to the Gulf of Guinea. Here the birds turn inland and reach the southern edges of the Sahara by early March, perhaps the most dangerous part of their migration as they face the vast and ever-widening expanse of the Sahara. With high temperatures, few feeding opportunities and prevailing northerly winds, many die in the effort.

Those who survive reach north Africa in a weakened state but press on past the Atlas Mountains and over the Mediterranean into Europe. Here they face a different (and mindless) danger as they run the gauntlet of trigger-happy shooters in Portugal, Spain, Italy and Malta. Millions of migrant birds are slaughtered each year despite "protection" in European legislation. Irish birds who survive this barrage eventually reach northern France and travel out across the sea to reach southern Ireland. It is a journey of over 9,500 km and is achieved in just two months.

This year I have been fortunate to follow the life cycle of Irish swallows for a documentary for RTÉ Radio 1. I learned that the males arrive at the breeding site first to establish a territory with females usually arriving about two weeks later. Once paired, the birds either repair an old nest or build a new one.

It has long been known that swallows return to the same area each year. To establish if this was indeed true, I visited a nest in Edenderry, Co Offaly, where Declan Manley, a qualified ringer, had caught and placed a small metal ring on an adult male in the summer of 2002. The ring has a unique series of codes and numbers. The same male returned in 2003 and paired with a female which Declan also caught and ringed. This summer both birds returned.

It is truly astonishing to think that since 2003, these two birds have independently flown to South Africa and back. Not only have they returned to Ireland, but they have found their way back to the same barn in Offaly and re-used their old nest. The male has now reached this nest site for three consecutive years!

It is a feat of navigation which is still not fully understood by scientists. Swallows weigh little more than 20 grams and have brains no bigger than a pea. Yet, they can store migration routes of over 9,500 km and use the sun, stars and familiar landscapes to find their way. It is even believed that migrant birds can store and memorise the unique odours of their breeding grounds and may even be capable of seeing the magnetic fields of the earth.

While the achievements of adult swallows are indeed noteworthy, the journey of young birds is even more remarkable. When young swallows first leave the nest, they spend time in the immediate area, roosting with other young swallows in reed-beds or maize fields each night.

During this period they learn the landscape of the region before, at less than three months old, they begin their own first long-distant migration without the benefit of experience and knowledge.

It is truly an incredible journey for young swallows which, in just five short months, will see them emerge from an egg in an Irish barn to arriving in South Africa. Sadly, it is estimated that the mortality rate for young birds may be as high as 70 per cent.

Why then do they face into such dangers to birds migrate? The answer is simply food. Most summer migrants require a plentiful supply of insects and each year a largely untapped and almost endless supply of insects is available in Europe. By late autumn, however, this supply is decreasing and birds need to leave for the south to survive.

I consider myself privileged to have been given a glimpse into the world of Irish swallows. I have watched them nesting in summer, seen young birds grow from helpless, bald chicks just five days old to fully fledged and independent swallows. I have watched the harsh landscape of the Sahara unfold beneath me as I flew to South Africa. There, in the Western Cape Province, I saw the first swallows arrive into their wintering grounds in late September.

I know that I will look with even greater awe at the first swallows I see next spring.

Eric Dempsey is a professional bird guide, broadcaster, and author of the Complete Guide to Ireland's Birds (Gill & MacMillan). He runs the Birds of Ireland News Service www.birdsireland.com

Swallows - from Egg to Africa, presented by Eric Dempsey, produced by Derek Mooney, will be aired on RTE Radio 1 tomorrow at 7.02 pm. Photo above: Anthony McGeehan