Vulture species which migrate for long distances may soon be spreading a fatal disease far and wide, conservationists say.
By Alex Kirby
BBC News Online environment correspondent
The disease, thought to be caused by a virus, has killed thousands of vultures in India, Pakistan and Nepal.
The vultures' decline is steep (Image: Guy Shorrock/RSPB)
The affected species do not migrate much, and so the disease has not travelled very far.
But scientists say another vulture species is highly vulnerable, and could cause havoc.
The disease, which makes the birds lethargic and causes their heads to droop, is usually fatal within a month.
It is currently affecting three closely related and largely non-migratory species: the Indian white-backed, the long-billed and the recently identified slender-billed vultures.
But researchers think a fourth close relative, the Eurasian griffon vulture, could succumb and spread the disease to new areas and to more vulture species.
Using satellites, they recently tracked two birds from their wintering grounds in northern India to previously unrecorded breeding sites in Mongolia.
Dr Debbie Pain is head of international research at the UK's Royal Society for the Protection of Birds (RSPB).
She said: "The vultures affected so far do not migrate over great distances, and this may have helped to limit the spread of the disease.
"However, everyone is very anxious to know whether it can be passed to extremely close relatives like the Eurasian griffon vulture."
Dr Andrew Cunningham is head of wildlife epidemiology at the Zoological Society of London (ZSL).
He said: "Since the outbreak of the disease in India, we have witnessed an increase in the number of Eurasian griffon vultures spending the winter there. We believe it is very likely the disease will spread to them."
That would leave vultures throughout Europe, Africa and the Middle East at risk within a few years.
The disease continues to take a heavy toll in India. The Indian white-backed vulture used to be thought possibly the commonest large bird of prey in the world.
But surveys between 1991 and 2000 showed its numbers had fallen by more than 95% in less than a decade, enough to classify it as critically threatened.
Dr Pain told BBC News Online: "The three affected species are declining by 25% a year - it's massive, and we're incredibly worried.
Once very common, vultures are now becoming rare (Image: Asad Rahmani)
"All three belong to the Gyps genus. So do the Eurasian griffon and another close relative, the Himalayan griffon.
"We've attached satellite tags to the two Eurasian birds which are now in Mongolia, and to two Himalayan birds.
"One of those seems not to have moved far at all, and with the other we're not getting a good signal, so we think either the tag's fallen off or the bird is dead.
"The trapping and harnessing is done by our other partner, the Bombay Natural History Society (BNHS).
"To carry the tag you have to use a flexible harness that's capable of expanding, because vultures can eat several kilos of meat at a sitting.
"Sometimes they gorge themselves so much they can barely take off."
Using British funding, the three partners are preparing to start monitoring and tagging vultures in the Middle East in October. The work will begin in Georgia, Yemen, Iran and Jordan.