But the researchers warned that the death rate from diclofenac was still too high and more needed to be done to eradicate illegal use of the drug to prevent the number of birds being poisoned creeping back up.

The study, led by Britain's Royal Society for the Protection of Birds, looked at more than 4,500 liver samples, mainly from cattle and water buffalo carcasses, at 21 sites across India.

Some 10 to 11 percent of samples from 2004-06 -- shortly before and after India banned veterinary use of the drug -- contained the painkiller, which causes a fatal kidney ailment in vultures called visceral gout.

But diclofenac was found in only 5.6 percent of samples from January 2007 to December 2008 and the concentration of the drug had also declined, said the study, published in the May edition of the peer-reviewed journal PLoS One.

Based on the results, the scientists estimated that the number of vultures expected to be poisoned per meal would have changed little between 2004 and 2006.

But by 2007-2008 estimated death rates "had slowed appreciably", said the study, entitled "Effectiveness of Action in India to Reduce Exposure of Gyps Vultures to the Toxic Veterinary Drug Diclofenac".

Conservationists working to save the vulture in South Asia are likely to welcome the study's findings.

Three species of the bird are found on the Indian sub-continent -- the oriental white-backed vulture, the long-billed vulture and the slender-billed vulture, collectively known as gyps vultures.

Their numbers plummeted by as much as 95 percent after diclofenac was introduced in the mid-1990s to treat colic in cattle, whose carcasses are left in the open after they die and are a food source for the scavengers.

A 2007 study by the Bombay Natural History Society conservation group estimated that there were only about 11,000 white-backed, 1,000 slender-billed and 44,000 long-billed vultures left in India.

The RSPB scientists said their findings were "encouraging" but continuing deaths from diclofenac were "of major concern" as it suggested the drug was being used illegally.

A similar situation "probably exists" in Nepal and Pakistan, they added.

"Only a very low proportion (less than one percent) of ungulate (hoofed mammals) carcasses is required to contain lethal levels of diclofenac in order to account for the rapid pre-ban population declines of gyps vultures," they said.

Even though it is now illegal to import, manufacture, sell or use diclofenac for veterinary purposes in India, a human version of the drug is still available, which may be being used to treat animals.

The scientists said more work was needed to prevent diclofenac being replaced by other painkillers like ketoprofen that have a similar toxic effect on vultures, and to promote drugs such as meloxicam, which are less poisonous.

The full study can be found at: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0019069

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