By Roland Pease
BBC science correspondent
A widely used livestock pain killer could endanger the survival of vultures around the world, researchers suggest.
Once very common, vultures are now rare in India (Image: Asad Rahmani)
The rapid decline of Indian vulture populations has been blamed on the use of the drug diclofenac to treat inflammation in cattle.
A new study for the British Royal Society for the Protection of Birds says diclofenac is highly toxic to other vulture species.
It warns they could be poisoned when feeding off contaminated carcasses.
Oriental white-backed vultures (Gyps bengalensis) were once the most common large bird of prey anywhere in the world.
Flocks of them could be seen across India; but over a matter of years, they have become almost extinct.
Oriental white-backed vultures are critically endangered (Photo: rspb-images.com)
The long-billed vulture (Gyps indicus) and the slender-billed vulture (Gyps tenuirostris) have also seen dramatic declines.
In early 2004, the painkiller diclofenac was identified as the cause by a US-led team.
Extremely cheap to buy, it has been widely used in South Asia to treat cattle for conditions like lameness, or mastitis - inflammation of the udder.
The trouble was that vultures were feeding off carcasses of dead, treated animals, and the drug was destroying their kidneys.
The new study, led by Dr Deborah Pain and published in Biology Letters, shows the drug is just as deadly to other closely related vulture species which are not so widespread.
The research centred on the Eurasian griffon (Gyps fulvus) and the African white-backed vulture (Gyps africanus).
The Eurasian vulture may be particularly at risk, as juveniles migrate into northern India, where dead cattle are left unburied.
But the study will alert conservationists across the world to the peculiar risk posed by diclofenac.
The researchers fear that more distantly related birds may be equally endangered, and that substitutes for diclofenac might be similarly toxic. They are currently seeking safe alternatives.
The loss of these scavenging birds - the world's natural rubbish collectors - could damage the quality of the environment.