The veterinary drug blamed for killing South Asia's vultures should be banned now, conservationists say.
Conservationists fear the birds may be headed for extinction (Photo: rspb-images.com)
In the past 10 years, population losses of more than 95% have been reported in three raptor species across many areas of the Indian sub-continent.
Strong evidence points to the livestock painkiller diclofenac, which vultures consume when they eat a carcass.
A new study in the Journal of Applied Ecology shows how little exposure is needed to knock back vulture numbers.
UK scientist Dr Rhys Green, of the Royal Society for the Protection of Birds and the University of Cambridge, the lead author of the new paper, said: "Our study indicates that diclofenac poisoning is the main cause - possibly the only cause - of these vulture declines, which are among the most rapid ever recorded for any wild bird.
"Time is running out if we are to save these species.
Governments, drug companies, vets, livestock owners and conservationists should act together now to solve this problem."
In the field
The three species are the Oriental white-backed vulture (Gyps bengalensis), the long-billed vulture (Gyps indicus) and the slender-billed vulture (Gyps tenuirostris).
All three are now classed as critically endangered.
The birds succumb to kidney failure and visceral gout when they eat a fallen animal that has been treated with diclofenac. Early signs that the raptors are affected can be seen from the way they hang their heads down to their feet for long periods.
The link between the drug and the dramatic fall in raptor numbers was established earlier this year by a US-led team.
Dr Lindsay Oaks and colleagues found high residues of diclofenac in dead vultures in the field.
They were also able to produce similar patterns of disease in experimental vulture colonies fed the drug either directly or via carcasses of buffalo or goat that had been treated with diclofenac.
In the latest research, British zoologists drew up a complex computer model that factored in vulture demography, eating habits, percentage of dead livestock likely to be available for these scavengers and the amount of residual diclofenac likely to be found in the carcases of buffaloes and goats.
The model showed that a surprisingly small proportion of livestock carcasses contaminated with the drug - less than 1% - is sufficient to cause the rapid declines in vulture populations observed in India, Pakistan and Nepal over the past 10 years.
The study also found that the proportion of dead vultures with symptoms of diclofenac poisoning is close to that expected if this was the sole cause of the declines.
Diclofenac is a non-steroidal anti-inflammatory drug that has been in human use for pain and inflammation for decades. The veterinary use of diclofenac on livestock in South Asia has grown in the past decade.
Vultures have an important ecological role in the Asian environment, where they have been relied upon for millennia to clean up and remove dead livestock and even human corpses.
Conservationists say a captive breeding programme will be essential to supplement vulture numbers while diclofenac is removed from the supply chain.