Quotes of the Day

Think you're a frequent flyer? Then talk to the whitethroat, a common warbler found throughout much of Europe and western Asia, which migrates on average an incredible 3,417 miles each year, wintering in sub-Saharan Africa. (Americans, by contrast, fly about 2,000 miles each year per capita.)

But that transcontinental migration is about to get even longer, thanks to global warming. In a new study in the Journal of Biogeography, scientists at the British universities Durham and Cambridge found that migration flights undertaken by warblers in Europe and Asia could be extended by as much as 250 miles by the end of the century. That's because as temperatures rise, the habitats of birds like the whitethroat, which breed in Europe, will need to shift farther north to more hospitable climates. But the birds' wintering grounds in Africa appear unlikely to shift northward — for reasons that still aren't clear — leaving the birds facing longer migrations. (See pictures of the effects of global warming.)

"That's bad new for these birds," says Stephen Willis, an ecologist at Durham and the study's lead author. "All that added distance is a serious threat." (See the new age of extinction.)

The whitethroat is hardly alone in clocking a killer flight schedule. Among other birds of its type — like the Subalpine warbler, the Orphean warbler and the Barred warbler — annual migrations exceed 1,500 miles, sometimes over the Sahara. It might seem that another 200 miles tacked onto a several-thousand-mile journey wouldn't be too taxing. But for the estimated 500 million birds that migrate annually from Europe and Asia to Africa, surviving the journey is already difficult enough. Migrating birds — some of them as small as your fist — pack on body weight to stock fuel for the flight, sometimes doubling in size before they leave. Certain birds even shrink their internal organs to make themselves more energy-efficient. Migrating species today also have to contend with the gradual destruction of the wetlands and other oases they might use as refueling stops along their flight path. Adding 250 more miles to the trip could be devastating. "These little birds push themselves to the limit of their endurance to fly these distances," says Willis. "Anything that makes these trips longer could be the difference between making it and not making it."

Willis' study, which was funded by the Natural Environment Research Council and the European conservation charity RSFB, is a future projection of the effect of climate change on migratory birds, but it is already being felt today. In previous studies, Willis and his colleagues found that birds like the Dartford warbler — which generally breed in the warmer areas of Western Europe — are increasingly being spotted in Britain, even though the island was thought to be too cold for them. (The U.K. is blessed with an energetic corps of amateur ornithologists, which means scientists there have a wealth of data on bird sightings.)

If birds are already responding to climate change and shifting northward, it's more evidence that temperatures are on the rise and that global warming is not just a problem for tomorrow.

See pictures of species on the brink of extinction.

See TIME's special report on the environment.

• Bryan Walsh
• British researchers predict that climate change will add hundreds of miles to the annual migrations of millions of warblers, which could be devastating for the little songbirds