They are the true jet-setters. Twice a year they set off on journeys most only dream of. Many cross the equator and still more switch continents, following the sun. Some go from one polar region to the other.
These are avian migrants, whose seasonal movements are only now beginning to be properly appreciated for the truly epic proportions they assume.
One particularly remarkable traveller is the little bar-tailed godwit, no bigger than a feral pigeon, which last year astounded all with its heroics when it went on a nine-day, non-stop flight of 10 200km - without a tailwind - from New Zealand's North Island to Yalu Jiang at the northern extreme of the Yellow Sea, between China and the Koreas. Followed with the help of a tiny satellite-tracking device, it went on to cause even more astonishment with a longer - in time and distance - non-stop return flight.
Consrvationists the world over are beginning to realise, to their consternation, just how hard our industrialised world has been making life for these long-distance travellers.
Through the ages, migratory birds have had, imprinted in their DNA, flight charts the staging posts where they could rest and feed before undertaking the next leg of their journey. But when arriving there, tired and hungry, they now increasingly find these spots to have been turned into building sites, crop fields, or timber yards.
Prof Phil Hockey, lead editor of the authoritative 7th edition of Roberts Birds of Southern Africa, says the consequences can be catastrophic. Many of the birds must cross vast, inhospitable stretches of land or sea, "and by the time they arrive at a stop-over, they pretty much run on empty".
He describes how the tragedy is increasingly being enacted in north Africa's Sahel, the semi-arid zone that separates tropical central and western Africa from the Sahara.
On the southward journey, birds have to cross the enormous desert. But when they arrive at their staging posts, exhausted and hungry, they find the region's evolving farming economy has left no food for them. "They die in their millions."
Northbound birds, knowing they have to cross the desert without the hope of finding food there, need to feed up properly in the Sahel. But because of the destruction of their habitats in the region, it takes them longer to stock up.
Such can be the delay in their marathon migration that by the time they reach their breeding grounds, they cannot find a mate, or too little time is left to rear their young before the snow comes, or the season has already changed so much that their usual foodstuffs are no longer freely available.
Until about 15 years ago, says Hockey, the concern was with breeding and non-breeding sites at the end of these migrations. But scientists now realise that the problem lies as much on the birds' flight paths.
Bird migrations are too complicated to break down but he estimates at least 150 of Southern Africa's more than 950 bird species are migrants. This underscores the need to have them looked after wherever they go. The new understanding Hockey refers to explains the welcome new phenomenon of legislation and treat
ies between countries that are aimed at making conditions less onerous for avian air traffic.
It also explains why conservationists are so eager to get countries to gain a better understanding of the true magnificence of bird migration.
One way of acknowledging the wonder of avian migration has been to declare an International Migratory Bird Day, which is this weekend (May 10). It is celebrated mainly in the northern hemisphere, where special events are staged to welcome back the many species that had left the cooling south for the warming climes of Europe, Asia and North America.
But plans are afoot to have similar celebrations in South Africa when the birds eventually get back here.
Peter Sullivan, BirdLife South Africa's chairman, says November 16, by which time most of the migrants are back, has been provisionally earmarked as Welcome Home the Migrants Day. The details will be discussed at a BirdLife Council meeting on June 14, but the general idea is that clubs around the country should think up their own ways of marking the occasion.
One event already in the pipeline is to have diplomats from Europe attend a function at Mount Moreland in KwaZulu-Natal to celebrate the vast numbers of Barn (or European) swallows gathered in the area's reed beds. They are part of an estimated 100 million of the birds that return every year from breeding in Europe for the warm season in Africa.
BirdLife International's partner organisations in Europe run a Spring Alive campaign which over their spring months has many people, but especially children, record their first sightings of barn swallow, common cuckoo, common swift and white stork. Participants get a Spring Alive bracelet on sending an e-mail or letter reporting a sighting to their national BirdLife organisation.
A major initiative to ensure safer flyways between Africa and Europe and Asia is a project called Wings Over Wetlands (Wow). It aims to protect wetlands, vital stop-overs for water birds on their intercontinental flights.
A collaborative effort by organisations such as BirdLife International, Wetlands International, United Nations conservation agencies and local partners, it fosters international collaboration along the African-Eurasian flyways and provides practical advice on wetland conservation through community mobilisation, management, ecotourism, wetland restoration, control of invasive species and transboundary management.
Leon Bennun, director of science, policy and information at BirdLife International, says the project is helping to safeguard the amazing sight of the waterbird migrations for future generations.
But a study has found that populations of shore birds wintering in south-eastern Australia have plummeted by 79% over the past 24 years. The key cause is thought to be loss of suitable feeding habitat at staging sites, particularly wetlands.
Prof Richard Kingsford is quoted as saying, "The wetlands and resting places that they rely on for food are shrinking virtually all the way along their migration path, from Australia through Indonesia, the Philippines, Malaysia and up through Asia into China and Russia."
China's Yellow Sea illustrates the crisis. It is a refuelling site for 36 species which are estimated to come to about three million. But it is also home to 600 million people, whose growing demands are destroying the tidal feeding grounds.
Migratory birds face similar difficulties on the other side of the world, where industrial development is threatening the vast Canadian boreal forest that forms a vital component in a chain of sites that run all the way down to South America.
A "Save Our Boreal Birds" campaign has been launched with its website saying nearly a third of the land has been allocated for activities such as oil and gas exploration, mining and logging. "In Ontario alone, 44 000 migratory bird nests were lost in 2001 due to logging."
One of the most effective ways of getting countries to look after habitats and share in the protection of migratory species is through Important Bird Sites, a worldwide project that uses globally agreed criteria to identify, monitor and safeguard a network of critical sites for birds.
BirdLife International says the 2008 IUCN Red List for birds will be released on May 19. The four-yearly update is a global assessment of every bird species on Earth. The last list showed one in eight of the world's estimated 10 000 bird species to be at risk of extinction.