Winging it! Marvels of migration

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Every year, some one-and-a-half billion birds migrate between Europe and Africa. Even in the age of satellite tracking, much about this extraordinary phenomenon remains a mystery to science. Why do they do it? How do they find their way? With World Migratory Bird Day approaching, Mike Unwin takes a closer look at these feathered nomads and their miraculous journeys.

StorkTAThe European or barn swallow has a truly extraordinary strategy for surviving the harsh British winter. Flocks gather in marshlands during autumn and, when nobody is watching, each bird slips down the reed stems and burrows deep into the muddy ground. Here they hibernate all winter, emerging only in spring to fly around the countryside once more.

OK, so that’s nonsense. Nonetheless, it was once one of several outlandish theories by which scientists tried to explain why certain European birds seemed to appear and disappear at different times of year. Others included redstarts ‘transmuting’ into robins and barnacle geese turning into, you guessed it, barnacles. But is the truth – that the swallows make a 9000-mile flight to South Africa then fly all the way back again six months later – really any less bizarre?

The more we learn about these tiny, feathered voyagers, the more mind-boggling their achievements become. And of course the swallow – so emblematic of migratory birds – is nothing unusual. Indeed, it is among some 4000 of the world’s 10,000 or so bird species that make regular seasonal migrations, with at least 1800 of these doing it long-distance.

Today, though, migratory birds are in trouble. To celebrate their miraculous feats and draw much-needed attention to their plight, the weekend of 10–11 May is World Migratory Bird Day (WMBD), a global initiative organised under the umbrella of the United Nations Environment Programme (UNEP). This event began in 2006 – the year avian flu gave bird migration such a bad press – and last year more than 350 separate awareness-raising events were held in 88 countries to mark the event.

Moving to survive
So why, you might wonder, do birds put themselves through the perils of migration, with its herculean demands upon energy and stamina? The answer is, quite simply, survival.

The risks of staying behind outweigh those of moving on. For the swallow, Europe’s summer may offer abundant food and long daylight hours for feeding a family, but winter is a cold, dark death trap. Conversely, southern Africa offers warmth and food year round, but there is too much local competition when it comes to breeding – not to mention an army of predators. Today most breeding birds in northern regions are migrants, whereas most species in the tropics – where food and weather are more reliable – tend to stay put.

And yes, migration is exhausting and hazardous. But for most birds, so is the daily routine of staying alive under any circumstances. Indeed, survival rates among sedentary species – those that stay put – are often lower than among migrants. Besides, evolution could not produce a system that made life more difficult. It simply doesn’t work that way. Natural selection ensures that only those adaptations that confer an advantage – whether in behaviour or anatomy – persist.

Different species complete their journeys in different ways. Many small birds lay down heavy fat reserves during a pre-departure feeding binge then strike out for their destination as directly as possible, flying mostly by night, when the air is calmer.

Larger birds, such as white storks, do not have the same capacity to sustain flapping flight. They have a more leisurely and circuitous journey, travelling in flocks, and stopping at feeding and roosting sites along the way.

Migrants’ navigational powers are truly astonishing: a young house martin hatched under the eaves of a cottage in the Lake District, for example, will migrate directly to central Africa after fledging and spend the next six months there before returning the following spring to those very same eaves. The mechanics of this still have scientists perplexed. We do know, however, that techniques vary from one species to another, and that the map of the night sky, an ability to detect the position of the sun via polarised light, a sensitivity to infrasound and the earth’s magnetic field, and an inherited memory of key landforms – such as river valleys – all play their part.

Route maps
Migrating birds travel along shared routes known as ‘flyways’. These reflect the prevailing weather conditions and topography of the areas they must overfly. Some follow coastlines, while others cross oceans or continental interiors. Most include pit stops – often wetlands or estuaries – where they can top up on food, water and rest. Many follow river valleys and other landforms, funneling through mountain passes, sea straits and other bottlenecks, where migrants congregate in large numbers.

The African-Eurasian flyway, one of three broad global flyways, accounts for the most species of any and carries an estimated 1.5 billion individual birds annually. The species that follow this route – migrating between Eurasia (the Western Palearctic) and sub-Saharan Africa – are known as Afro-Palearctic migrants. They include numerous songbirds, such as wheatears, warblers and swallows, as well as larger species such as storks and raptors, and a host of shorebirds and marine species that journey along the coasts.

Within this broad Afro-Palearctic flyway, each species follows its own route. The straits of Gibraltar, which offer the narrowest crossing of the western Mediterranean, see the passage of millions of birds during peak migration – larger species, such as honey buzzards, rising high on thermals before gliding down to the other side. Birds that take this route then work their way around the western Sahara to the Sahel region of west Africa before continuing to their eventual destination.

Species arriving from eastern Europe and Russia, such as red-backed shrikes, cross the Bosphorus, near Istanbul, then continue south down the Nile Valley into East Africa. Others, including cuckoos, cross the central Mediterranean, continuing their journey south right across the Sahara. A few eastern species, such as the Amur falcon, cross directly over the Arabian Sea from southern Asia.

Most shorebirds, such as turnstones, breed in the high Arctic and work their way around the east and west coasts – some stopping to spend the winter at various points along the way, others moving on as conditions dictate. The osprey, being a fish-eating raptor, also takes a maritime route, with most of those from northern Europe ending up along the West African coast. Not all birds arrive and depart by the same route. Some follow a ‘loop’ migration, returning by a different route from their outward journey in order to exploit seasonal food supplies or prevailing weather conditions.

Sand martins, for instance, fly to Africa over the western Mediterranean, passing to the west of the Alps, but return in a loop via the eastern Mediterranean, passing to the east of the Alps.

Bird migration in Africa is not confined to Afro-Palearctic migrants. Within the continent, a number of species make regular seasonal journeys between central and southern regions. Known as ‘inter-African migrants’, these include various species of cuckoo, bee-eater and kingfisher, plus such larger birds as Wahlberg’s eagle and Abdim’s stork.

There are also species – notably those of arid grassland regions, such as quelea and finch-larks – that move from region to region in response to rainfall. These movements, which respond quickly to optimum breeding conditions, are known as irruptive migrations.

Dangerous journeys
While we marvel at migration, there is no room for complacency. Migrants may have evolved to overcome the worst that nature can throw at them – the storms, the droughts, the predators – but they struggle to withstand the impact that our own species has made. Threats range from manmade obstacles such as power lines and gas flares to the hunters’ guns and snares that lie in wait across the Mediterranean and West Africa. Over five million migrants die each year in Malta alone.

Most serious, though, is the loss or destruction of habitat along flyways – notably the drainage of wetlands. Climatic factors also play a part: the desertification of the Sahel, due to overgrazing and climate change, is extending the desert crossing for many species on the Afro-Palearctic flyway.

Recent news about migratory birds has been grim. The UK alone has seen shocking declines in many Afro-Palearctic migrants. The turtle dove population, for instance, fell by 74 per cent between 1995 and 2009. The causes are complex – ecologically unfriendly farming techniques on the species’ European breeding grounds, uncontrolled hunting over its Mediterranean migratory route and habitat loss in its West African winter quarters all feature – but they reveal the central problem that all migrants face: that of living in many different homes. How can you protect a species in one place if it is blasted from the air before it even arrives?

This is why the priority for conservationists today is protecting flyways: establishing safe migratory corridors for birds. And to this end, research is under way. The RSPB’s Migrants in Africa project, for example, is working with local scientists and communities in Ghana and Burkina Faso to discover just how changing land use there affects wintering migrants. With this knowledge, conservationists can start working with governments and communities to devise strategies that make a difference.

Such projects are forging new connections: both between Britain and Africa, and among scientists and local people. Shared information means shared interests. Besides, whose birds are they anyway? Those of us in the UK might talk about ‘our’ turtle doves. But given that these birds spend less than five months on British soil, couldn’t the people of Ghana or Burkina Faso make an equal claim?

Turtle doves, of course, are not too worried about nationality. They were migrating back and forth between Africa and Europe long before we humans started carving up the map. If there’s one thing that migrating birds can teach us, it’s that some things go beyond borders and boundaries.

 

Migrant meals
Many predators feed on migrating birds. Eleonora’s falcon, however, is unusual in that its entire lifestyle is adapted to exploiting this food source. This fast-flying hunter of songbirds nests on rocky cliffs in the Mediterranean. Its chicks fledge in August, much later than most birds’, thus enabling the parents to pick-off returning migrant songbirds in order to feed their growing chicks. Once fledged, the falcons themselves migrate south to Madagascar.

 

Fatter means fitter
Many small migrants have the capacity to turn food quickly into fat, forming a layer beneath the skin that is converted into energy as they fly. The sedge warbler may double its weight from 10g to 20g in just one three- to four-week binge prior to departure, gorging largely on aphids and gaining three to four per cent of its body weight each day. This fuel allows it to undertake a single, non-stop 4000km flight to West Africa.

 

Migration miracles
• Migrating vultures can use thermals to reach astonishing heights. A Ruppell’s griffon vulture was recorded at a height of 11,300m above West Africa – 27 per cent higher than the summit of Mount Everest.
• A group of bar-tailed godwits captured by ringers in New Zealand had flown 11,000km non-stop from Alaska in just six days, without making landfall. It is thought these shorebirds were assisted by the jet stream.
• A young Manx shearwater captured from its breeding burrow in Wales was released on the US coast near Boston, thousands of kilometres from its wintering grounds. It was back at its burrow 15 days later.
• The Arctic tern is the only bird known to migrate between the Arctic and Antarctica, completing a 30-40,000km round trip, the longest known of any bird. Throughout its life it sees only summer.

 

Circumpolar songbird
The latest satellite tracking technology, using tiny 1.5g transmitters, has revealed the epic migration of the northern wheatear, whose round-trip of up to 30,000km between sub-Saharan Africa and its Arctic breeding grounds is the longest of any songbird. The species’ enormous breeding range extends from the eastern Canadian Arctic, across Greenland, Eurasia and into Alaska.
Amazingly, birds from all these regions migrate to spend the winter in the northern parts of sub-Saharan Africa. The Alaskan birds travel almost 15,000km each way, crossing Siberia and the Arabian Desert. The Canadian birds must cross the northern Atlantic Ocean, journeying to Africa via Greenland, Iceland and the UK. Scientists believe these extreme journeys have arisen as the bird has extended its breeding ranges east and west while remaining faithful to its Afro-Palearctic migration flyway.

 

Top 5 migration hotspots
Afro-Palearctic migrants are best seen in Africa from October to March, during the European winter. These are a few hotspots in which to look out:
Banc d’Arguin – National park on the coast of Mauritania that forms the world’s largest pit-stop for migrating shorebirds. An estimated 2.25 million waders winter here, including hundreds of thousands of curlew sandpiper, red knot, bar-tailed godwit and dunlin.
The Gambia – This tiny west African nation offers wintering quarters for numerous Afro-Palearctic migrants. Any patch of coastal woodland from October to March holds nightingales, turtle doves and numerous warblers. Meanwhile, curlew sandpipers, bar-tailed godwits and other northern waders flock along the shoreline, and ospreys plunge into the tropical surf.
Kruger National Park – This celebrated reserve lies close to the southernmost limit of many Afro-Palearctic migrants. October sees the arrival of huge numbers of European rollers, red-backed shrikes, European bee-eaters and barn swallows, while Montagu’s and pallid harriers arrive to quarter the savannah.
Luangwa Valley – Zambia’s premier national park is home to many inter-African migrants. Carmine bee-eaters (pictured above) arrive in September to throng the sandy riverbanks, while October rains also bring woodland kingfishers, red-chested cuckoos and other noisy travellers from central Africa.
Gibraltar   – In autumn, the narrowest crossing point of the Mediterranean witnesses the greatest migratory bird spectacle in western Europe. Honey buzzards, short-toed eagles, white storks and other large birds gather in huge numbers, rising on thermals on the Spanish side before crossing to Africa.

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