Do Vultures Have a Sense of Smell?

New World vultures, native to the Americas, and Old World vultures, native to Asia, Africa and Europe, aren't close relatives at all, but they look a lot alike. Members of both groups are large, with wide wingspans and bald heads and necks so that when they feed, fluid from decaying flesh doesn't gum up their feathers. Vultures release heat from their heads to maintain stable body temperatures. But Old World vultures are related to birds of prey such as hawks, falcons and eagles, and New World vultures are relatives of storks. Adaptations that occurred separately in both groups, called convergent evolution, account for features they have in common.

Among the seven species of New World vultures, some have sense of smell so sensitive that they can pick up the scent of a dead animal from more than a mile away. For example, the turkey vulture, also known as the buzzard, can hone in on a rodent decomposing under a pile of leaves. Other New World birds descended from slightly different family trees, such as the black vulture and king vulture, don't have the same well-developed sense of smell but compensate with a clever adaptation: From a distance as far as a couple of miles, they watch the behavior of their cousins as they sniff out carrion. Then they move in, drive the other vultures off, and eat the food themselves.

In many countries with large populations but poor public sanitation systems, vultures play an indispensable role in keeping environments free of dead animals and the disease-carrying bacteria and insects that quickly multiply on them. All 16 species of Old World vulture living throughout Asia, Africa and Europe have poor sense of smell, so they rely entirely on their eyesight to find food. For millennia, the relationship between vultures and humans has worked to mutual advantage. Vultures living around populated areas get plenty of food, and people don't have to dispose of the remains of animals that die naturally or have been butchered for their meat.

People living on the Indian subcontinent have always relied on vultures to help cleanse the environment. So in the 1980s, when people started noticing that vulture populations were declining, there was great concern. By 2004, when a veterinary anti-inflammatory drug called diclofenac was finally identified as the culprit, the birds were on the brink of extinction. Among Hindus, cows are considered sacred; in most places in India, killing them is illegal. When cows got old and feeble, out of compassion, people had been giving them diclofenac to ease their discomfort. This drug, still present in cows' tissues after death, is toxic to vultures. Recovery efforts are under way, but with 95 percent of vultures wiped out, success is far from assured. Among many other consequences of this mass die-off of vultures, cases of human rabies and anthrax are on the rise.