By Victoria Gill
Science and nature reporter, BBC News
The population of the Oriental white-backed vulture (left) crashed by 99.9%
A ban on the veterinary use of a painkiller in South Asia appears to be preventing vultures from being poisoned there, say researchers.
Many vultures have been killed by eating livestock carcasses that were contaminated with the drug.
The crash in vulture populations was so severe that the three endemic species are now threatened with extinction.
This study, published in the journal PLoS One, reveals the first signs that the ban has reduced vulture poisonings.
The governments of India, Nepal and Pakistan banned the use of diclofenac for livestock in 2006.
But by this time the oriental white-backed vulture population had crashed by 99.9% and populations of the long-billed vulture and slender-billed vulture had fallen by about 97%.
The researchers set out to assess the effectiveness of the ban by measuring the concentration of the drug in livestock carcasses.
They took samples from the livers of 5,000 of cattle carcasses. Some samples were gathered one year before the ban, some immediately after its implementation, and some between 2007 and 2008.
This revealed that the proportion of cattle carcasses in India contaminated with the drug declined by approximately 40% between 2006 and 2008.
In animal carcasses that were contaminated, the concentration of the drug was significantly lower.
But, according to the RSPB, which helped fund the research, diclofenac manufactured for human use is still being used in India to treat cattle.
One of the authors of the research, Professor Rhys Green, a Cambridge University conservation scientist, said: "The ban on veterinary manufacture and sale of diclofenac appears to be having some effect, but the job is only about half done.
"Diclofenac use must be virtually eliminated, and not just halved, if vulture populations are to recover."
The drug, an anti-inflammatory, is used to reduce pain and swelling in injured and diseased cattle.
For religious reasons, it is not the practice in India to kill dying cattle to relieve their suffering. So owners give the painkiller to cattle and buffaloes when they are very close to death.
"Only then is there enough diclofenac left in the tissues of the dead animal to kill vultures," said Professor Green.
"An alternative legal drug, meloxicam, also relieves pain effectively and is safe to vultures.
"Its use is increasing, but it has not yet replaced diclofenac."
Munir Virani, from the Peregrine Fund, an organisation that supported research into the cause of South Asia's vulture decline, said the results were "incredibly encouraging".
"But," he added, "we must be extremely cautious about jumping the gun to say that vulture populations are on the road to recovery."