Scientists believe they have identified the main cause behind the catastrophic decline seen in Asian vulture numbers.
G. bengalensis is now critically endangered (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.
Lindsay Oaks' research team has now shown the birds are dying after eating the carcasses of livestock treated with the common veterinary drug diclofenac.
Dr Oaks, backed by The Peregrine Fund, reports his work in Nature magazine.
"This discovery is significant in that it is the first known case of a pharmaceutical causing major ecological damage over a huge geographic area and threatening three species with extinction," the US researcher from Washington State University said.
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. Early signs that the raptors are affected can be seen from the way they hang their heads down to their feet for long periods.
Such has been the alarming decline in bird numbers that international organisations have pumped hundreds of thousands of pounds into research to track down the cause of all the deaths.
Now, Dr Oaks and colleagues have found high residues of diclofenac in dead vultures in the field.
They have also been able 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.
Other possible causes of death, such as poisoning by mercury or arsenic or infection by viruses, have been investigated and ruled out.
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.
The Nature report has led ornithological and other conservation groups to call for the immediate withdrawal of diclofenac from use.
"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," said Dr Munir Virani, a biologist for US-based Peregrine Fund, and who coordinated the massive field investigations across Nepal, India, and Pakistan.
"Their loss has important economic, cultural, and human health consequences."
One immediate impact has been the explosion in feral dog populations which have moved into areas no longer scavenged by vultures.
Britain has invested significant research time and money on the vulture problem through its Darwin Initiative for the Survival of Species.
Dr Debbie Pain, a research scientist at the Royal Society for the Protection of Birds, said: "In the 1980s, [Gyps bengalensis] was thought to be the most abundant large bird of prey in the world, but in little over a decade, the population has crashed by more than 99%, with the loss of tens of millions of birds.
"The decline of Asian vultures is one of the steepest declines experienced by any bird species, and is certainly faster than that suffered by the dodo before its extinction. If nothing is done these vulture species will become extinct."