Why do vultures circle dead stuff?
Dear Straight Dope:
Why do buzzards circle above dead stuff? My husband tried to tell me that they're just making sure their potential dinner is really dead. I'm more inclined to think it's a way of alerting the rest of the pack (flock? clan? club?) that the buffet line is forming. So, what's the story?
There may be some truth in both these ideas, Michelle, but they are probably not what's usually going on. When you see a vulture circling over that hunk of 'possum tartare on the Interstate, it could be trying to sniff out whether the carcass is still fresh enough to eat. "Rotten enough to gag a vulture" is more than just a saying. Contrary to popular belief, vultures like their food as fresh as they can get it.
The birds you see are probably turkey vultures, Cathartes aura. Colloquially called "buzzard," it is one of three species of New World vultures (family Cathartidae) found in the U.S. ("Buzzard" is a misnomer, the name properly applying to a European hawk related to the familiar red-tail of North America.) The other two U.S. species are the black vulture, Coragyps atratus, with a more southerly distribution than the turkey vulture, and the nearly extinct California condor, Gymnogyps californianus. The vultures' scientific names are quite appropriate--cathartes meaning "a purifier," and atratus, "dressed in mourning, as for a funeral." (The proper collective noun for a bunch of vultures is just flock, although wake has also been proposed.)
All three species feed mostly on carrion, but will sometimes kill injured or helpless prey. They are generally very cautious in approaching a prospective meal. Perhaps they sometimes circle to be sure that the animal is really dead, and thus no threat, but it's more likely that they are checking to make sure there is no potentially dangerous competitor, such as a coyote, lurking about.
The turkey vulture is one of very few birds with a well-developed sense of smell. It took a surprisingly long time to prove this, however. Several early investigators, James Audubon among them, tried to test their sense of smell by concealing extremely putrid meat as bait. When vultures showed no sign of being attracted, the researchers concluded that the birds were unable to detect the smell. In reality, turkey vultures have an astonishing ability to locate food by smell, it's just that they prefer fresh meat to rotten. In an experiment in Panama, a researcher put out chicken carcasses in the tropical forest, hiding them so they were not visible from the air. Vultures rarely found the baits on the first day, evidently because they were still too fresh to give off much odor, but located almost all of them by the second day, after they had started to reek a little. However, the vultures clearly preferred fresher carcasses to more rotten ones. The bacteria that cause putrescence give off toxins, and although vultures do have cast-iron stomachs compared to other animals, even they prefer not to deal with really nasty stuff if they can avoid it.
The black vulture is a different story--it can't smell worth a damn. While it is quite capable of locating its own food with its keen vision, more commonly it freeloads off the food-finding capabilities of the turkey vulture. When foraging, turkey vultures fly relatively low so they can detect scents from the ground. In contrast, black vultures often fly very high, where they can keep an eye on several turkey vultures soaring far below. When they see one start to descend on a find, they follow it. Black vultures are very aggressive and can usually chase a turkey vulture from its meal even though they are similar in size. They are also quite social, and often gather in large groups at overnight roosts. A bird that has found a carcass too big consume in one day may be followed by less successful roost mates seeking to share in the bonanza when it leaves the roost the next morning. However, they probably do not deliberately lead others (except offspring) to food, nor do they signal others by circling above it.
In the American tropics the huge king vulture, Sarcoramphus papa, adds another level to the story. The king, unlike its funereal cousins, has white body plumage, black wings and tail, and a gaudy red, yellow, blue, and purple head ornamented with bizarre wattles and carbuncles. Like the black vulture it lacks a sense of smell, and it flies even higher, where it can monitor many other vultures. It also probably follows turkey vultures to food, and is big enough to usurp a find from blacks. But the smaller species may sometimes benefit when a king arrives. Having relatively weak beaks, they are unable to tear through the tough hide of larger animals, and if it is intact they must resort to just plucking at the eyes, mouth, and, er, other orifices. The king instead has a massive beak that can easily open up a carcass. Although the king feeds first, once it has had its fill the others are able to feast as well.