There is a moment during the capturing of baby vultures when the human nose can be considered an asset. In the Bandhavgarh Tiger Reserve of central India, this moment comes for us atop a 100-foot-high cliff etched with natural ledges and carved crenelations of an ancient Hindu fort built into the cliff's sandstone face. These high niches are prime nesting habitat for long-billed vultures, but this year only a few of the great birds have returned to nest, and chicks are few and far between. When a pungent, three-day-old diaper smell wafts up to us, we peer down, and there, on a ledge 30 feet below us, lies an eagle-size chick in a messy nest of twigs.

One of the nestling's enormous parents wheels into view. We see its full seven-foot wingspan, the tawny plumage on the adult's back rippling in the updraft, its darker wing feathers splayed at the tips. The bird banks hard and alights on the ledge. It nudges the chick, opens its long bill and urps up supper.

"Uh-oh. Bad timing," Richard Wesley says.

"Yep," says Richard Cuthbert. "You'll be seeing that meal again."

Cuthbert is a biologist with the United Kingdom's Royal Society for the Protection of Birds. Wesley is taking a busman's holiday from his job managing the New Zealand Alpine Club. The third member of this cliff-side team is a Bombay Natural History Society biologist named Shanmugam Saravanan.

Wesley clips a cloth bag to his rock-climbing harness and steps over the edge of the cliff. The adult bird dives away. Wesley drops 30 or so feet to the ledge, scoops the ungainly chick into the bag and climbs back. A wine-dark fluid seeps from the bag. At this point in vulture catching, the human nose can be considered a liability. "Vulture chicks vomit up the contents of their crops when they're stressed," Cuthbert says apologetically. "Thought to be a defense mechanism. Rather an effective one."

If the bag's stench of twice-regurgitated carrion reinforces one's stereotypes about the repugnance of vultures, the chick that emerges from the bag dispels them. Up close, the baby is a beauty—the bare skin of its swan neck palest aqua, its pinfeathers a wild duck's browns.

The long-billed vulture, Gyps indicus, is one of three vulture species that serve as sanitation engineers in India, Nepal and Pakistan. For thousands of years, they have fed on livestock carcasses. As many as 40 million of the birds once inhabited the region. Obstreperous flocks of vultures thronged carcass dumps, nested on every tall tree and cliff ledge, and circled high overhead, seemingly omnipresent. In Delhi, perching vultures ornamented the tops of every ancient ruin. In Mumbai, vultures circled the Parsi community's hilltop sanctuary. Parsis, who are members of the Zoroastrian religion, lay their dead atop stone Towers of Silence so that vultures can devour the flesh. This practice, according to Parsi tradition, protects dead bodies from the defiling touch of earth, water or fire.

But across the subcontinent all three species of Gyps vultures are disappearing. Dead livestock lie uneaten and rotting. These carcasses are fueling a population boom in feral dogs and defeating the government's efforts to combat rabies. Vultures have become so rare that the Parsi in Mumbai have resorted to placing solar reflectors atop the Towers of Silence to hasten the decomposition of bodies. International conservation groups now advocate the capture of long-billed, white-backed and slender-billed vultures for conservation breeding.

That's why we're here. Cuthbert and Saravanan have permits to take eight long-billed vulture chicks from Bandhavgarh. (Young birds adapt more readily to captive conditions than adults, and once these birds can fly they're almost impossible to catch.) The recovery plan calls for a minimum of 25 pairs of each vulture species to be held in each of three breeding centers in northern India.

But these wild vultures are disappearing so fast—up to 99 percent of the population is now gone—that the captive-breeding goal is unlikely to be met. Many conservationists believe it's already too late for the Gyps vultures of the Indian subcontinent to survive in the wild.

It is an astonishing turn of events. "Just 15 years ago Indian Gyps vultures were thought to be the most numerous large raptors on the planet," Cuthbert says. "In a single decade they've undergone the most rapid population collapse of any animal in recorded history."

Villagers in northern india were the first to notice. People started complaining about livestock carcasses lying around, rotting and attracting dogs. In 1996, in a town north of Delhi, Asad Rahmani, a wildlife biologist at Aligarh Muslim University, saw an item in the daily newspaper: "Where Are the Vultures?" the headline asked. That's odd, Rahmani thought. He checked the municipal carcass dump and found that there did seem to be fewer vultures.

India has more livestock than any country but China, "yet we are principally vegetarian," says Rahmani. "We keep cattle and buffalo primarily as dairy animals." Out in the countryside, when an animal dies, a skinner trundles it away in a pushcart, dumps it beside the road, flays it and leaves the carcass there. In urban areas, haulers take dead animals to official dumps. "It has always been the vultures' job to dispose of the flesh," Rahmani says.

As many as 100 vultures may feed on a single cow carcass, stripping it clean in 30 minutes. Two thousand, 3,000, even 10,000 vultures swarmed the larger dumps in the early 1990s, the huge birds lapping at carcasses with their leathery tongues, thrusting their narrow heads neck-deep to reach internal organs, tussling over choice gobbets of meat. Year after year, Rahmani says, five million to ten million cow, camel and buffalo carcasses disappeared neatly down the gullets of India's vultures.

Rahmani, who became the director of the Bombay Natural History Society (BNHS) in 1997, organized the first of several meetings about the problem. Were biologists in other parts of India noticing a decline in vulture populations? Vibhu Prakash, a biologist with the BNHS, had documented a sharp drop. In a 1987 survey at Keoladeo National Park in the state of Rajasthan, Prakash had counted 353 breeding pairs of the white-backed vulture, Gyps bengalensis. Following up nine years later, Prakash found just 150 pairs. The next year there were only 25. By 1999 the Keoladeo vultures were gone.

Prakash couldn't tell what was killing them. The problem certainly wasn't a shortage of food—there were thousands of livestock carcasses at a dump in Rajasthan. Nor was it habitat degradation: prime nesting trees were still standing. Though pesticides were being used in agricultural areas, scientists thought the chemicals an unlikely culprit. "Birds that feed on other birds and on fish accumulate pesticides," Prakash says. "Birds that feed on mammals usually don't." Nevertheless, the researchers couldn't rule out the chemicals.

Pathologists could test for pesticide residues in dead birds—if suitable ones could be found. But in a place where daytime temperatures routinely exceed 100 degrees, fresh carcasses were hard to come by. Many of the birds died as they perched high in trees, and their carcasses, entangled among the branches, decayed where they hung. Those that ended up on the ground were dispatched by dogs, jackals and other scavengers. Prakash eventually found two vulture carcasses worth testing. One bird had keeled over as Prakash was observing it through binoculars, and he raced to find its carcass before the dogs did. The second had nested for years in the garden of an American living in Delhi. She had read about how rare the birds had become, and when she found one dead on her lawn, she phoned the BNHS.

Prakash rushed the two fresh carcasses to Haryana Agricultural University in the northwest Indian city of Hisar. A pathologist slit them open—and almost dropped his scalpel. The internal organs were covered by a whitish paste of uric acid crystals, a condition called visceral gout. The birds' kidneys had failed. But why?

Viruses can cause kidney failure. And the epidemiology of the mysterious die-off suggested an infectious disease caused by a virus or bacterium. "Vultures feed in groups, they nest in flocks, and they fly long distances," Prakash says, all behaviors that facilitate the transmission of disease. Also, the malady appeared to be spreading into Pakistan and Nepal. There are eight Gyps vulture species in Asia, Africa and Europe, with overlapping ranges. The virus, if that's what it was, had already killed more than 90 percent of India's vultures. It could kill Europe's and Africa's vultures as well.

In early 2000, BNHS, the Royal Society for the Protection of Birds (RSPB) and the U.S. Fish and Wildlife Department, which had funded Prakash's surveys, collaborated with the Zoological Society of London and the Idaho-based Peregrine Fund to help determine what was killing the vultures. The agency scientists knew they would have to find more carcasses and run sophisticated virology, bacteriology and toxicology tests on them.

But there was a snag. India strictly limits foreign researchers' use of indigenous biological materials. In the 1980s and '90s, foreign corporations prospecting in India had patented basmati rice, turmeric, black pepper extract, and the chemical in the neem tree used for cleaning teeth and controlling crop pests; as a result, Indians watched foreign corporations earn royalties on products from plants that Indians considered part of their natural heritage. In response, the government passed laws controlling access to genetic material and restricting the shipping of biological samples abroad. In order to get permits to export tissue samples for analysis, the vulture researchers would have to prove that the work couldn't be done in India. Frustrated, Prakash, Rahmani and their British colleagues decided to build a pathology lab and vulture-care center in India.

The Peregrine Fund took a different approach. "Pakistan is right next door to India. It allows the export of tissue samples. So we set up shop there," says Munir Virani, a Peregrine Fund biologist. In Multan, in central Pakistan, Virani found everything he needed: an ultra-low temperature freezer for storing samples; a source of liquid nitrogen for shipping them to the lab of a Washington State University microbiologist, Lindsay Oaks; a partner, the Ornithological Society of Pakistan, which helped furnish permits; and three still healthy, wild breeding colonies with a total of 2,500 pairs of white-backed vultures.

The only thing Virani and Oaks couldn't seem to find was fresh vulture carcasses. "Thirty million dead vultures, you'd think we could find at least one," Oaks says. Three weeks' searching yielded only four dead birds. Back at Washington State, Oaks found visceral gout in these carcasses, but after running scores of tests, the scientists found nothing to explain what had caused the condition. Political upheaval in Pakistan in the wake of the September 11, 2001, terrorist attacks kept Virani and Martin Gilbert, a Scottish veterinarian, from returning to Multan later that year. Instead, Muhammad Asim, an accountant for the Ornithological Society of Pakistan, took over the carcass hunt. His team of university students, carrying coolers of dry ice, searched at night and early in the morning to find carcasses not yet fried by the sun. Oaks tested the dozen carcasses they found for infectious viruses and bacteria, heavy metal poisoning, pesticides and nutritional deficiencies. But all he found was gout. The next year they continued the search; that season's carcasses, too, showed signs only of gout. "Well, I can tell you what they're not dying of," Oaks groused to Virani in early 2003. Yet by then an estimated 90 percent of Pakistan's Gyps vultures and 95 percent of India's had died.

Oaks, Gilbert and Virani then began to focus on another idea. "The food source for these birds is almost all domestic livestock," Oaks says. "We knew it all along but it hadn't clicked. And the one thing we hadn't looked at was what goes into livestock."

There's a little pharmacy on almost every block of almost every town in South Asia, and Multan is no exception. "You can go in and say, ‘My cow's not eating, what can I give her?' and the pharmacist will root around under the counter and find something, and off you'll go with it," Oaks says.

Asim and his students tore around Multan, making a list of every drug and nutritional supplement sold for use in livestock—35 or 40 products. Which ones were cheap, potentially toxic to kidneys and new to the market? There was one, Oaks found—a nonsteroidal anti-inflammatory drug that had been used as a painkiller for decades in the West, but had only recently been licensed for veterinary use in India, Pakistan and Nepal: diclofenac.

Oaks checked his vulture samples. All 28 birds with gout now tested positive for diclofenac, and all 20 birds with no gout (killed by gunshot or other causes) tested negative. "That was a very strong association," Oaks says, sounding pleased with the understatement.

Reproducing the effects in live birds would help clinch the diagnosis. Though Pakistanis, most of whom are Muslim, do eat beef, they rarely eat buffalo and never eat donkey. The carcasses of the latter two are the primary food for Pakistan's vultures. An aged buffalo slated to become vulture food was dosed with diclofenac, slaughtered and fed to captive vultures. All the birds died within six days; their necropsies showed visceral gout.

Oaks and Virani received those results just as they arrived at a world conference on vultures in Budapest in May 2003. Euphoric, they presented their findings to the assembled experts. This is no virus, they said; the vultures of the Indian subcontinent are being poisoned by a pharmaceutical drug given to domestic livestock, whose carcasses are subsequently consumed by vultures.

But "How?" asked members of a stunned and skeptical conference audience. How could a prescription pharmaceutical drug reach tens of millions of vultures across nearly two million square miles of South Asia? Many scientists and conservationists, along with journalists from around the world, remained unconvinced.

Nita Shah, a wildlife biologist at BNHS, has studied Indian ungulates for two decades. Nomadic herders carry a sophisticated pharmacopeia, Shah says, thanks to the availability in India of cheap drugs. A 1972 law allowing Indian companies to reverse-engineer patented drugs spawned a gargantuan pharmaceutical industry. And though India superseded that law in 2005 with one that upholds international patents, some 20,000 pharmaceutical companies duke it out for market share in the nation today, selling drugs for a fraction of what they cost in the West. In India, diclofenac is manufactured in veterinary doses by at least 40 companies.

Herders use diclofenac to treat pain, inflammation and fever in their animals. "Western India especially is covered with invasive thorn bushes, which cause a lot of small injuries," Shah says. "And then maybe the animal can't keep up with the group, or is more subject to predation. So a herder learns these tricks of the trade when his migration takes him near urban centers, and then knowledge of any new drug spreads by word of mouth."

Asim surveyed 84 pharmacies, clinics and village shops in the Punjab and Sindh and found veterinary diclofenac at all of them; 77 sold it daily. The drug is highly effective—it'll speed a cow's recovery from an inflamed udder so it can be milked the next day, or cool the heat in an ox's sore hip so it can pull a plow. Not all animals recover, of course. Some die within a day or two, regardless of treatment. Their skinned carcasses are left for vultures.

How many freshly dosed animals would have to die to account for 30 million or more dead vultures? Surprisingly few. A Cambridge zoologist calculated that only 0.1 to 0.8 percent of livestock carcasses would have to contain diclofenac to kill vultures at the rate observed. Prakash and Cuthbert collected tissue samples from almost 2,000 livestock carcasses across the Indian cowbelt. Almost 10 percent contained diclofenac.

With this last piece of data, BNHS and RSPB considered the case closed. In February 2003, they converted the pathology lab and vulture-care center in Haryana to a long-term captive-breeding center.

In March 2005, Prime Minister Manmohan Singh of India directed that veterinary use of diclofenac be phased out within six months. Six months stretched to 14, but this past May, India's drug controller general directed pharmaceutical companies to halt production and sales of diclofenac within three months. Nepal banned the manufacture and importation of the drug in June 2006, and Pakistan did so in September. An alternative drug, meloxicam, is now being made by a dozen or so pharmaceutical companies. It appears to be harmless to vultures.

The ban will help, Cuthbert says, but vultures take five years to reach reproductive age, and lay only one egg per season. "Even if we got rid of all the [remaining] diclofenac tomorrow, the recovery would take decades." Meanwhile, cow carcasses are mounting up all over northern India. They are "a time bomb waiting to explode," Munir Virani says.

At the dusty red wasteland that serves as the town carcass dump for Kota, in eastern Rajasthan, seven men flay a quartet of fresh cow carcasses. The men laugh and joke, and a festive atmosphere prevails despite the rotting meat, the sick-sweet stench of carrion and hair-raising shrieks and snarls from dogfights. Crows, mynas and Egyptian vultures pepper the grotesque windrows of bone.

It appears these smaller vultures are being poisoned too. Cuthbert and Prakash have recently documented significant declines in Egyptian and red-headed vultures. There's been no toxicity testing on them, nor has anyone surveyed the populations of steppe eagles, kites and other, smaller avian scavengers, but the scientists speculate that those birds are also being poisoned, now that the big Gyps vultures no longer elbow them away from livestock carcasses.

Diclofenac doesn't hurt the dogs. (No one knows yet why the drug kills birds but not mammals.) At the dump, 50 or 60 yellow-brown dogs tear at carcasses. Under every mesquite bush, sated dogs lie curled, asleep. "Yes, the dogs are many now that the long-necked vultures are gone," a skinner says. India doesn't cull dogs because of Hindu and Buddhist prohibitions on taking life. In the past, starvation and disease kept dogs in check. With vultures so vastly reduced in number, dogs have more than enough to eat; their population increased from 22 million in 1992 to 29 million in 2003, the last year for which figures are available. India's official human death toll from rabies is the world's highest—30,000 deaths annually, two-thirds of them caused by dog bites. In recent years, the government has made rabies vaccines more widely available in rural areas, but rabies deaths aren't decreasing at the rate they should be because the unvaccinated dog population is growing, according to rabies experts.

Public health officials say it's likely that India's rat population is growing too, sharing the bounty of abandoned carcasses with feral dogs, and raising the probability of outbreaks of bubonic plague and other rodent-transmitted human diseases. Livestock diseases may increase too. Vultures are resistant to anthrax, brucellosis and other livestock diseases, and helped control them by consuming contaminated flesh, thus removing reservoirs of infectious organisms. Some municipalities are now resorting to burying or burning carcasses, expending precious land, firewood and fossil fuels to replace what Rahmani calls "the beautiful system nature gave us."

Time is not on the researchers' side as they race to capture vulture chicks before the birds die in the nest, poisoned by contaminated carrion. The odds of any young vulture living to breeding age in the wild is almost zero. The team has plucked its quota of eight long-billed vultures from the Bandhavgarh cliffs in three days, and Saravanan has hurried the birds off to the breeding center in Pinjore, north of Delhi. When I ask Cuthbert what the likelihood is that the breeding program will achieve its goal of capturing 450 vultures, he shakes his head and turns away.

Compared with long-billed vultures, white-backed vultures are more widely dispersed and harder to find—they nest in trees rather than cliffs, so the remnants of their population could be almost anywhere. On a sweltering afternoon our jeep heads out of Bandhavgarh National Park's far gate. Soon the odor of rotting diapers envelops the jeep. We all shout to the driver to stop, and he jams on the brakes. We leap out and trace the familiar stench down a bank to a copse of tall trees. But there is no vulture nest. Just a rotting cow carcass, unattended.

Hours later, thanks to a sharp-eyed local forest warden, we do find a nest—a haystack of twigs in a tall tree. Cuthbert and Wesley toss a line over a branch, jousting amicably over who gets to climb. A chick makes the question moot when it flaps nonchalantly over to join its parents on a neighboring tree. This chick has fledged; they'll never catch it now. We watch the youngster in silence. It has escaped capture and a life of tedium in a breeding center—and fled to certain death.

Seattle-based Susan McGrath, who wrote about cormorants in the February 2003 issue, specializes in environmental subjects.