Tiny warbler at risk from longer African migration

They are some of the world's most remarkable and improbable journeys – vast odysseys across desert, mountain and sea by creatures often no bigger than a Mars bar. But the annual flights of Europe's migratory birds to and from sub-Saharan Africa are set to get even longer.

Climate change, shifting the breeding range of many European bird species northwards, is likely to lengthen the migrants' marathon journeys substantially, in some cases by hundreds of miles, a new scientific study predicts. The added distance is likely to make what are already hazardous and chancey long-distance flights even more risky, with possible fatal consequences for many birds.

One example is one of Britain's most charming summer visitors, the whitethroat, a small bouncy warbler pouring into Britain by the thousands right now after journeying from its winter quarters in the Sahel, the arid zone south of the Sahara. The whitethroat may face another 300 miles or more on the average length of its Africa to Europe trip by the end of this century. This added distance would be a considerable threat, said Stephen Willis of Durham University, leader of the research team which produced the study.

The researchers have carried out the first examination of how global warming may, by the end of the century, affect the "climate space" of European breeding birds on their wintering grounds in Africa – the climate space being the climatic range in which they can comfortably exist. This is a follow-up to a study last year, extensively reported in The Independent, which showed that many birds' climate space in Europe would shift towards the north as temperatures rose.

But the new study shows that for the bird group studied – the Sylvia family of warblers, of which the whitethroat (Sylvia communis) is one – the African climate space does not move in some cases, while the European one does, meaning that the migratory journey gets longer.

A staggering 5 billion birds of nearly 200 species are estimated to migrate to Europe and Asia from Africa every spring, with some weighing less than 10g undertaking the voyages between the two continents to find food and suitable climate. Their journeys may be 4,000 miles long, or in the case of birds such as the swallow, which migrates from South Africa, as many as 6,000 miles.

To travel such immense distances the birds have to put on large amounts of weight as fat; they even shrink the size of some of their internal organs to become more fuel efficient. Some species must even double their weight. They also need a miraculous sense of navigation, and a lot of luck to survive drought, predators and hunters.

"From 2071 to 2100, nine out of the 17 species we looked at are projected to face longer migrations, particularly birds that cross the Sahara desert," said Dr Willis. "This is bad news for birds like the whitethroat."

Professor Rhys Green, of Cambridge University and the RSPB, another of the researchers, said: "These tiny birds make amazing journeys, pushing themselves to the limits of endurance. Anything that makes those journeys longer or more dependent on rare and vulnerable pit-stop habitats used for refuelling on migration could mean the difference between life and death."

African odyssey: Warblers' marathon journeys

Three more of the warbler species face increased distances on their migratory journeys from sub-Saharan Africa as long as, or longer than, those faced by the whitethroat, the study shows. These are birds of central and southern Europe which are not often found in Britain.

* The first is the subalpine warbler, Sylvia cantillans, a very pretty songbird with an orange breast set off by a white "moustache" which is found in southern Europe, especially around the Mediterranean coast. It faces an average increase in its journey of about 470 miles.

* The second is the orphean warbler, Sylvia hortensis, which looks rather like a larger version of the blackcap familiar to English bird lovers, which has a very similar range to the subalpine. It faces an increase in its journey of about 340 miles.

* The third is the barred warbler, Sylvia nisoria, a fairly inconspicuous bird of an ashy grey-brown colour, which migrates to breed in central and eastern Europe. It faces an average increase in its annual journeys of nearly 600 miles.

The researchers only studied 17 bird species in total and it is quite possible that many more migrants to Europe from Africa may face longer annual journeys.

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