A wider ban of the veterinary drug diclofenac and more captive breeding centres are the only way to save the birds found mainly in India, the researchers said in the Journal of the Bombay Natural History Society.

India banned manufacture of the veterinary form of the anti-inflammatory in 2006, but a version formulated for humans is still used to treat livestock, the researchers said. When the vultures feed on carcasses they ingest the drug, which shuts down their kidneys and kills them within days.

"The ban on diclofenec production for veterinary use was an excellent first step," Vibhu Prakash, a researcher at the Bombay Natural History Society and colleagues wrote. However, this action is insufficient on its own to save these species."

The birds are critical to the ecosystem and for human health in India because they are the primary means of getting rid of animal carcasses in the nation of some 1.12 billion people, added Andrew Cunningham, who worked on the study.

Their demise is has led to a sharp increase in dead animals around villages and towns, which has boosted the numbers of disease-carrying rats and rabid stray dogs, he said.

"This is a direct consequence of the decline of the vultures," Cunningham, a veterinarian at the Zoological Society of London, said in a telephone interview.

The researchers counted vultures in northern and central India between March and June last year. They surveyed the birds from vehicles along more than 160 sections of road totalling 18,900 kilometres.

The study followed four previous counts and was the first since 2003. The researchers warned that all three species could dwindle down to a few hundred birds or less to the verge of extinction in fewer than 10 years.

The researchers believe the number of oriental white-backed vultures in India could now be as low as 11,000 from tens of millions in the 1980s. Populations of the long-billed vultures have likely dropped to 45,000 while only an estimated 1,000 of the slender-billed species remain, they said.

The dodo was hunted to extinction barely 100 years after it was discovered in the 16th century.