by Stephen Moss

Migrating birds such as the garden warbler and whitethroat will face longer journeys because of climate change, experts warned today.

A team of scientists led by Durham University has demonstrated that while the birds' breeding ranges are likely to shift northwards, their wintering areas will not, thus increasing the length of their journeys by up to 250 miles. The study, published in the Journal of Biogeography, has serious implications for many of the birds returning this month to Britain to breed.

The research team used computer simulation models similar to those used by weather forecasters to analyse how climate change might affect the migration patterns of European Sylvia warblers.

Every year these tiny birds - some weighing as little as 12g - travel thousands of miles northwards from their African winter-quarters to breed in Europe and Asia. Up to 500 million birds undertake this epic journey to take advantage of the long summer days and glut of insect food in temperate latitudes.

But as climate change leads to rises in spring and summer temperatures, some of these long-distance migrants are responding by shifting their breeding ranges further north, making their return journey each spring even longer than before. In future, when they return to Africa in autumn, the predicted southward extension of the Sahara Desert may also eventually increase their travel distance.

Dr Stephen Willis of Durham University, who co-ordinated the study, stressed the problems this additional mileage will pose. "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. The added distance is a considerable threat," he said.

Different birds follow different migration strategies, with some species covering the distance in short hops. Others, such as the sedge warbler, fatten up and almost double their body weight before making the journey across the Sahara and Mediterranean Sea in a single leap. But whichever way they travel, longer journeys will make enormous demands on their energy resources, which they may not be able to meet.

Professor Rhys Green of the RSPB, who co-authored the research paper, believes that this increase in distance - though relatively small compared with the total journey the birds undertake - is likely to cause major problems. "Anything that makes those journeys longer or more dependent on vulnerable pit-stop habitats used for refuelling on migration could mean the difference between life and death," he warned.

There is evidence that short-term climatic effects have caused problems in the past. For example, the prolonged drought in the Sahel Zone of western Africa during the late 1960s led to a massive 90% fall in the British population of whitethroats in a single year, from 1968 to 1969. Although numbers later recovered, the species remains highly vulnerable to sudden environmental change.

Some species, however, may already be beginning to adapt their migration patterns as a result of climate change. The chiffchaff, which normally migrates to Iberia and North Africa rather than crossing the Sahara, now regularly overwinters in Britain, especially in the milder areas of the south and west. Meanwhile the German and Austrian populations of the blackcap, one of the species followed in the study, now migrate west instead of south to spend the winter here in the UK.

If other species can follow suit by changing their migration patterns they may be able to survive. "Some species may be able to adapt and change, for example by adopting shorter migration routes, if they can find enough food at the right time," said Willis.

"Bird migrations are incredible feats of stamina and endurance but, as temperatures rise and habitats change, birds will face their biggest challenge since the Pleistocene era."