Finding Unseen Carrion
In 1979, David Houston did a study on the efficiency of Turkey Vultures finding carrion by smell. The study was conducted in the tropical forests of Barro Colorado Island, Panama. Setting out dead chickens from fresh to four days old, he discovered that Turkey Vultures within tropical forests rely almost exclusively on their sense of smell to locate carrion. During the study he found that Turkey Vultures only located a small percentage of the fresh carcasses. On the other hand, the percentage of Turkey Vultures coming down to carcasses that were four days old, was also low. Conversely, they quickly found the carcasses that were one to three days old, even below the tree cover, and even if completely covered with a layer of dried leaves.
So why did it appear that Turkey Vultures were not as efficient in finding four day old carcasses that surely had a strong odor? Was it because the older, stronger smelling baits produced a different odor that the vultures could not detect as easily? Or did they prefer meat that wasn’t as badly decayed?
To answer the question, nine sites were each bated with two carcasses. In each site one of the two baits would be 24 hours old, with the other being 120 hours old. Houston found that when vultures were given a choice between fresher and older carcasses they prefer the fresher. It was observed that a high percentage of the time Turkey Vultures even tended to reject older, more rotten meat. In the final analysis it was concluded that it was the decayed condition of the carrion that failed to draw the vultures in, rather than the stronger smell.
As a side note unlike Turkey Vultures, Black Vultures do not have the olfactory abilities to located carrion by smell. While studying Turkey Vultures, Houston noted that Black Vultures in the area were unable to locate the hidden carcasses on their own. However, they would follow Turkey Vultures down through the tree cover to feed on the bait.
Finding Buried Carrion
The ability of Turkey Vultures to locate carrion hidden from view is also documented by Kirk and Mossman. In 1998, they buried a woodchuck carcass and covered it with about 3.93 – 5.91 inches of soil, then tamped the soil down by foot. Next, the area was tilled leaving no visual signs of the burial. Two days later a Turkey Vulture was observed circling over the field. When the workers and the author of the report left the field for a lunch break, the vulture was observed immediately descending and landing directly upon the burial site. The vulture then scratched away the soil covering the woodchuck and began to feed.
Although it is possible the Turkey Vulture observed the burial, it is unlikely. Turkey Vultures in the area normally return to their roosting trees one to three hours before sunset. The woodchuck was buried at dusk and there were no Turkey Vulture roosts near the tilled field.
Furthermore, in 1998 Whitaker and Hamilton reported that Turkey Vultures are known to dig up and consume prey caches of red foxes and other large predators.
The Smell of Ethyl Mercaptan
As early as the 1930s, concentrations of ethyl mercaptan began to be added to natural gas pipelines. This odor helped in locating natural gas leaks. The odor of ethyl mercaptan is given off naturally by corpses. As a result, this addition to gas lines attracted Turkey Vultures. Making use of this observation, the presence of Turkey Vultures can alert field petroleum engineers to possible gas leaks.
In 1964, K.E. Stager conducted a set of experiments with ethyl mercaptan in which he confirmed that Turkey Vultures could locate odors associated with ethyl mercaptan, even when there was no visible object associated with the odor. The observation and studies concluded that the odor of ethyl mercaptan rising off of carrion appears to be a contributing factor in alerting Turkey Vultures to the presence of a potential meal.
Listen to this short Bird Note on Turkey Vultures and Gas Pipe lines: https://www.birdnote.org/show/turkey-vultures-and-gas-pipelines
Physical Structure and the Sense of Smell
In the 1960s, Kenneth Stager conducted anatomical studies of the Turkey Vulture’s brain. His studies showed that Turkey Vultures had an extremely large olfactory bulb in their brains. He did not, however, have microscopic proof that the vulture’s larger olfactory bulbs bestowed any advantage to the bird’s ability to smell better than other birds.
In 2017, an important new study on the anatomical structure of the vulture was published. (See report here: https://www.nature.com/articles/s41598-017-17794-0) Taking a microscopic look at the brain, the study proved that the olfactory bulb in Turkey Vultures is significantly larger relative to the brain volume. Also, compared to the brains of Black Vultures, Turkey Vultures have twice as many mitral cells as Black Vultures. Mitral cells pass information to other parts of the brain.
The researchers also conducted comparison studies of Turkey Vulture’s brains against 32 other birds species from 10 different orders. They found that, “In absolute numbers, the Turkey Vulture has more mitral cells than any other species measured.” This new information is significant, as it proves that the olfactory structure in a Turkey Vulture’s brain does give them an advantage in their ability to smell more efficiently.
Though these behavioral and anatomical structure studies bring the debate about the Turkey Vulture’s use of smell to a close, there is still a lot to learn on the subject. Gary Graves, a co-author of the anatomical study, states that in the future he hopes to explore whether olfactory receptors in Turkey Vultures differ from other birds, humans, or mammals.
Graves is also interested in discovering what vultures are actually detecting in the odor of death. Although ethyl mercaptan appears to be one source, there are hundreds of chemicals in the odors rising off of carrion. It has not been determined if Turkey Vultures are keying in on one particular scent, or possibly a number of odors. Graves states, “It’s important to keep studying the massive birds.” They “continuously clean up all sorts of things that could cause human and livestock illnesses.” He also comments that Turkey Vultures save highway departments millions of dollars by scavenging road kill.
Photo of Suli , the Draper Museum Raptor Experience’s Turkey Vulture, downloaded from the Raptor Experience’s Facebook Page, https://www.facebook.com/pg/DraperMuseumRaptorExperience/posts/?ref=notif
Turkey Vulture Soaring by Tony’s Takes, Attribution-NonCommercial-NoDerivs 2.0 Generic license, https://www.flickr.com/photos/tonystakes/
Black Vulture and Turkey Vulture by Dawn Beattie, Attribution 2.0 Generic License, https://www.flickr.com/photos/pics4dawn/
Turkey Vulture Landing by Tony’s Takes, NonCommercial-NoDerivs 2.0 Generic license, https://www.flickr.com/photos/tonystakes/
Group of Turkey Vultures on deer carcass by TwoWild, Attribution-NonCommercial-NoDerivs 2.0 Generic license, https://www.flickr.com/photos/rvewong/
Head Shot in Six Mile Cypress Slough by Dennis Church, Attribution-NonCommercial-NoDerivs 2.0 Generic liceense, https://www.flickr.com/photos/dfc_pcola/
Turkey Vulture dining with hopeful Magpie nearby by Tony’s Takes, Attribution-NonCommercial-NoDerivs 2.0 Generic license, https://www.flickr.com/photos/tonystakes/,