In 1994 the Indian government approved diclofenac, a cheap anti-inflammatory painkiller, for veterinary use in cattle. The drug worked. Sick cows recovered. Lame cows walked again. Hindu communities, which do not slaughter cattle and therefore must keep them alive in some form whether they are productive or not, found that diclofenac dramatically reduced suffering in old and arthritic animals. Within a few years it was being administered, in varying degrees of regulation, to most of the country’s livestock.

There was a side effect that nobody had thought to test for. The drug, which was harmless to cows and to humans and to dogs and to almost every animal it was tested on, was acutely lethal to vultures.

When a treated cow died and was left, as Indian cattle traditionally are, for vultures to dispose of, the birds that fed on the carcass developed kidney failure within days. They could not flush the drug. Three species of South Asian vulture - the White-rumped, the Long-billed and the Slender-billed - crashed so fast that by 2007 their populations had fallen by more than 95 per cent. The White-rumped Vulture had been one of the most numerous large raptors on earth in the 1980s, with tens of millions of birds. By the time anyone understood what was happening it was Critically Endangered.

This is the part of the story that gets told. The next part does not get told as often, and it is what this piece is about.

When the vultures collapsed, the carcasses did not. Indian livestock continues to die at the same rate as before. Without vultures to strip the bones in hours, the dead cattle now took weeks. They rotted in fields and at the edges of villages. Two things happened in slow sequence.

First, the feral dog population exploded. India already had the largest free-roaming dog population on earth. The carcass surplus fed an additional surge, which a 2008 paper in Ecological Economics by Markandya and colleagues at the Indian Statistical Institute estimated at roughly seven million additional dogs over the decade.

Second, rabies cases rose. India already had the highest rabies death toll of any country, around 20,000 a year, almost all of them from dog bites. The post-vulture surplus dogs were not vaccinated, not contained, and present in numbers no public health system could keep up with. The same paper estimated 47,000 additional human deaths between 1992 and 2006 attributable to the rabies surge that the vulture collapse had set off. The economic cost ran to tens of billions of dollars.

A bird that almost no one had thought about as anything but a scavenger on the margins of human attention turned out to be load-bearing.

This is the part of the carrion-eating-birds story I want you to hold in your head while we count the species. The cleanup crew is not optional. The cleanup crew is doing public-health work that nobody else does. The cleanup crew is, in most cultures, the bird that gets the worst press.

There are roughly forty species of bird globally that live mostly or entirely on carrion. The world’s vultures - twenty-three species split between New World and Old World, two evolutionary lineages that converged on the same job from completely different starting points - are the specialists. Their adaptations are extreme. Bald heads, because feathers in a body cavity carry rot. Stomach acid at around pH 0.7, more acidic than household drain cleaner, which dissolves anthrax and botulism toxin and cholera. Vast soaring wings that let them quarter hundreds of square kilometres without flapping. A wholly different sense of smell in the New World genus Cathartes - the Turkey Vulture can find a deer carcass under closed-canopy forest by scent at a kilometre’s distance.

Outside the specialists, the corvid family - crows, ravens, magpies and jays - does the opportunistic share. Ravens in Yellowstone follow wolves through deep snow because wolves leave carcasses they cannot pack into. American Crows have learned which roads produce roadkill at which times of day. The world’s largest seabirds - the Marabou Stork, the Giant Petrel - take seal placentas and dead whale tissue on the same evolutionary logic as the inland vultures.

Then there are the part-timers, who do not advertise. The Bald Eagle on the national seal of the United States is, statistically, a scavenger as much as a hunter. Studies in the Pacific Northwest find Bald Eagles drawing close to half their winter calorie intake from carrion: spawned-out salmon, beached marine mammals, deer killed by cars. Benjamin Franklin’s much-quoted opinion that the Bald Eagle is “a bird of bad moral character” who steals fish from hard-working Ospreys turns out to be empirically defensible. The bird is, more days than not, a vulture in a national costume.

What you call this set of birds depends partly on which one you grew up watching and which one your local culture taught you to like or dislike. The Andean Condor has been a sacred bird in Andean cultures for at least three thousand years. The Bald Eagle is on the back of a US quarter dollar. The Turkey Vulture is, for most North Americans, the bird that makes you slow the car for a second to confirm it is not a hawk. The species-level prestige rankings have almost nothing to do with what the birds actually do.

After the diclofenac story broke, the international raptor conservation community moved fast. India banned veterinary diclofenac in 2006. Pakistan and Nepal followed. A small number of breeding centres in Haryana, Pinjore and West Bengal pulled the last birds out of the wild for captive breeding. The Saving Asia’s Vultures from Extinction programme began releasing captive-bred birds back into safer ranges in the 2010s. The White-rumped Vulture, which fell by 99.7 per cent, has been growing slowly since 2012. The recovery is fragile and ongoing. Two decades after the crash, the most numerous vulture in the world at the start of the story is still measured in low tens of thousands.

A second drug, aceclofenac, metabolises to diclofenac inside the cow’s body and produces the same vulture mortality. It is still legal in some South Asian markets, despite repeated calls from raptor biologists for a ban. Several European countries approved veterinary diclofenac in 2013 for use in livestock in the Balkans, where Griffon Vultures, Cinereous Vultures and Egyptian Vultures still range. Conservation groups are now fighting in 2020s Europe a version of the fight that was lost in 1994 India. The lesson does not seem to scale.

What you do with this, if you are a person at a feeder in a North American suburb, is small. You probably do not see vultures often. You may see a Turkey Vulture in a soaring kettle on a warm afternoon, six or seven birds rising on a thermal over a hillside, and you may or may not register what the bird is doing for the airshed beneath it. You may see a Bald Eagle on a beach pulling at a dead seal and assume the eagle is being unworthy of his image.

The right response to either bird is the one nobody is taught. Nod at the bird. Thank him for clearing the carcass. Recognise that you are watching a species that no human society has ever managed to do without and that several have learned, the hard and expensive way, what happens when it stops showing up.

The vultures of India were doing public-health work. They were not on the payroll. They were not represented in any government department. They had no constituency and no lobby. When the painkiller came in, nobody asked them what they thought, because nobody had thought to ask.

Then they were gone, and the country counted the cost in human lives, and someone had to ask.