You have to feel sorry for vultures. For animal campaigners they are a difficult case. Other, more photogenic, slightly less sinister creatures may gain the world's sympathy at the drop of a hat, but raising money to save the world's most proficient scavenger is a different matter.

As far as the Asian vulture is concerned, however, the situation is now urgent. Asian vultures may be ugly, but soon, if current trends continue, their unprepossessing appearance will be consigned to history.

The population of the oriental white-backed vulture, predominantly native to India, is dwindling at a rate of 40 per cent a year, making it the fastest declining wild bird in history. Their numbers have plummeted by 99.9 per cent since 1992. Indeed, its slide to extinction may be more rapid than that of the dodo.

Once believed to be the most common bird of prey in the world, the oriental white-backed vulture population in India is now a thousandth of what it was just 16 years ago. Scientists estimate numbers of the birds have plummeted from tens of millions in the 1980s to 11,000 currently in the wild.

The bird, which is also found in Pakistan and Nepal, is just one of three species of Asian vulture which could face extinction within 10 years if urgent action is not taken, according to a report from the Zoological Society of London (ZSL) and the Bombay Natural History Society.

Chris Bowden, of the Royal Society for the Protection of Birds (RSPB) said the speed of the bird's demise had been phenomenal. "It's extraordinary that a bird which was thought to be the most abundant bird of prey in the world is now facing extinction. It used to be a feature of visiting India that these birds weren't just in the countryside, but also in urban areas.

"As a bird organisation, we're used to fighting for little brown birds that nobody even sees. But this is a totally different situation; this is a keystone species that plays a huge role in preserving the environment."

Zoologists are calling for a ban on an anti-inflammatory livestock drug they say has caused the decline. The drug, called diclofenac, is passed to the birds when they pick at the carcasses of animals recently treated with it. When ingested by the vultures – even in small quantities – it can cause fatal kidney failure.

The acute poisoning can kill a vulture often within just two days. Because the vultures often gather in numbers of up to 60 to feast on one carcass, it would take only a handful of carcasses containing the drug for dozens of vultures to be killed off in a very short space of time.

Manufacture of the drug was banned for veterinary purposes in India in 2006, but it remains widely available, and conservationists now want a total prohibition of its sale. The report's lead author, Dr Vibhu Prakash, of the Bombay Natural History Society, said: "Efforts must be redoubled to remove diclofenac from the vultures' food supply and to protect and breed a viable population in captivity."

Oriental white-backs are not the only vultures under threat from diclofenac. The population of long-billed and slender-billed vultures has also declined by 97 per cent since 1992, leaving their numbers at approximately 45,000 and 1,000 respectively. All three species of vulture are now classified as "critically endangered" by the World Conservation Union, which is the highest risk category short of being extinct in the wild.

The research published today was based on a survey of almost 19,000km of roads in northern and central India last year. Many of these roads were near protected areas, so vulture numbers may be even lower than the report estimates.

Conservationists are setting up breeding programmes in captivity to try to ensure the species is not wiped out altogether. So far, three of these centres have been set up in India, but altogether they house fewer than 200 vultures, and conservationists say this is not nearly enough.

A further two centres have recently become operational in Nepal and Pakistan, but these are similarly too small to tackle what has become an urgent issue.

The urgency comes not just from concern at the loss of another species, but the devastation its absence is already causing to the ecosystem the bird once dominated. In the past, millions of vultures would use their 7ft wingspan to patrol India's skies, keeping a watchful eye for dead or dying animals. Yet beneath the macabre image of circling scavengers was an invaluable service to India's living. The health consequences of the vultures' exit from a vital place in the ecosystem have reverberated across the country.

Dr Andrew Cunningham, principal investigator and co-author of the study, explains: "These vultures were the main scavengers in the Indian ecosystem. They removed all the carcasses of livestock and wildlife very effectively. They would strip an adult cow of its flesh within an hour or so, leaving nothing but bones to be bleached in the sun. But that's no longer happening.

"Indian society had evolved to rely on the vultures to be their disposal units for carcasses. But within the past 10 years, this disposal system has been switched off, so you've got large numbers of carcasses building up.

"Other scavengers have moved in and taken advantage of the newly available food. But these interlopers – stray dogs and rats, among others – aren't as effective as the vultures. What's more, they bring with them disease.

"When a rat or a dog eats a carcass, they don't remove all the flesh, meaning piles of rotting meat are left behind which can contaminate water. Farmers – who were previously unaccustomed to dealing with carcasses – have not been quick to adapt to the change. They have tried to burn or bury the remains, but they lack the equipment, money or experience to get rid of the bodies efficiently."

India's Zoroastrian community – known as Parsis – are also feeling the loss of the once-abundant birds. For centuries they have relied on the vultures to dispose of their dead, leaving bodies out in the open to be consumed by the birds.

It is against the Parsis' faith to bury, burn or submerge their dead in water, which means vultures provided them with a hygienic solution. An entire human carcass can be consumed to the bone within a matter of hours by a large group of vultures. But now that their numbers are dwindling, half-eaten bodies can be left for days, leaving the community susceptible to disease.

These are not the only catastrophic consequences for human health. Previously, if a cow died of anthrax, its flesh would be quickly consumed by vultures, so the deadly spores would not have time to spread. The rate of consumption by dogs or rats is not nearly so fast.

As the number of vultures declines, the number of feral dogs in India has risen dramatically, thanks to the extra food available. These stray dogs are increasingly hunting in packs, thus posing a threat to humans as well as wildlife. Diseases such as rabies are on the increase: India now has the highest rate of human rabies in the world, partly due to the increase in feral dogs.

Dr Cunningham says the crisis in vulture numbers would be noticeable even to a passing tourist in India. "They used to be almost like pigeons – you'd see them gathered on rooftops in Delhi. But they've all gone now."

He thinks the true fall-out from the loss of these birds is yet to be seen. "Who knows what more effects there could be further down the line?" he asks. "When you take out such an important part of an ecosystem there will be long-term consequences which we may not even be able to imagine."

The bird that was killed by Western civilisation

The dodo's rapid demise in the mid to late 17th century was predominantly due to human contact.

The three-foot tall flightless birds once waddled around the Indian Ocean island of Mauritius free from predators. It is possible they had been living there for millions of years, until the late sixteenth century when Portuguese sailors and Dutch colonists landed.

The birds were so unaccustomed to humans that they simply walked straight up to them. Exactly how many of the birds existed when sailors first arrived on the island is unclear, but it was this foolish curiosity that got many of them killed.

It is said to have earned the birds, which resembled oversized pigeons, the name "dodo", meaning "simpleton" in Portuguese.

Not only did they become an easy-to-catch food for the settlers, the birds also fell victim to changes in their previously untouched habitat.

Forest clearing by the settlers destroyed the birds' natural home. Species brought in by the settlers, such as pigs, goats and rats became competitors to the birds, as well as predators. All this, plus, as recent archaeological research suggests, flash floods on the island, finally killed the bird off. It became extinct so quickly after its discovery that for generations it was believed that the dodo was an entirely mythical creature.

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