By Alex Kirby BBC News Online environment correspondent

Scientists anxious to track the movements of a rare bird say atomic analysis of its feathers may reveal its migration route.

The bird is the aquatic warbler, a globally endangered species chiefly confined to eastern Europe. The warblers' main breeding sites are known, but no-one has yet found where they spend the winter.

That knowledge could improve the prospects for conserving the birds.

The warblers breed mainly in Poland, Belarus and Ukraine, with smaller numbers in Lithuania and Hungary. Much of Eastern Europe has lost key bird habitats since 1945, largely because of drainage schemes.

Ornithologists think there may be some unknown breeding populations, possibly in Siberia.

The birds are believed to winter in the wetlands of west Africa south of the Sahara desert, but only one warbler has ever been caught there.

Permanent record

Greater certainty would make it possible to give the species better protection against the main threats to the African wetlands - drainage, encroachment by farming, and the spread of the desert.

Scientists at the UK's Royal Society for the Protection of Birds (RSPB) are working with colleagues in several other European countries on the problem.

Polish birds go off on their own

They are concentrating on a study of stable isotopes - atoms of the same type but with different masses - and trace elements in the birds' feathers.

The RSPB says: "When feathers grow, concentrations of elements and ratios of stable isotopes deposited within them reflect the elemental and isotopic profiles of the environment in which they were grown, and these remain constant throughout the lifetime of the feather.

"As aquatic warblers grow their tail and wing feathers in Africa, feathers taken from adult birds on migration or at their breeding grounds will tell us something about their African wintering quarters.

"The element and isotopic signatures of these feathers should be similar to those in feathers of resident African warblers occupying similar areas.

"Feathers collected from recently fledged warblers on their natal grounds in Europe, and from young birds on autumn migration, will allow us to identify the breeding origin of the migrating birds."

The team has already collected feathers from a range of surrogate species around Lake Chad in Nigeria, and is looking for further sites where warblers may be wintering.

Feathers unruffled

Dr Debbie Pain of the RSPB told BBC News Online: "All inert tissue, like feathers, hair and nails, will show what isotopes were present when it was formed.

"With this research, all you need is to snip off the end of one tiny feather - we're not engaged in a mass feather-pulling exercise.

There may be unknown breeding sites

"So far we've found that birds from Poland winter somewhere different from those hatched in Belarus and Ukraine.

"Because it's fairly easy to catch juvenile birds flying out of Europe, it should be relatively simple to identify any new breeding grounds.

"But finding where the warblers winter will be harder, because it's more difficult to get feathers from surrogate species.

Antarctic puzzle

"We're also hoping to discover whether all the birds migrate on the same route through France and Spain.

"If we find any whose feather profiles don't fit, we may find not only new populations but new migration routes as well.

"Isotopes tell you a lot about diet - they're a reminder that 'you are what you eat'.

"They're also being used to research what's happening to rockhopper penguins."

Images courtesy of the RSPB.