09/02/2026
In 1938 and 1939, Union Oil used turkey vultures to find leaks along a 42-mile natural gas pipeline in California. Workers injected ethyl mercaptan into the line, then patrolled the route looking for vultures circling or sitting on the ground. Where the birds gathered, they found the leaks.
Natural gas itself has little odor, so utilities add strongly scented sulfur compounds to make leaks detectable. Ethyl mercaptan, now usually called ethanethiol, has the rotten, sulfurous smell people associate with leaking gas. Similar sulfur compounds also occur in the odor released as animal tissue decomposes. To a turkey vulture, a leaking pipe could smell enough like carrion to deserve a closer look.
The Union Oil trick later caught the attention of Kenneth Stager, an ornithologist at the Los Angeles County Museum. At the time, scientists were still arguing over a question that had occupied naturalists for more than a century: were turkey vultures actually smelling carcasses, or simply spotting them from the air?
Stager built experiments designed to remove the visual clue. He concealed carcasses, blew air carrying carrion odor from hidden sources, and eventually used a compressed-air device to release ethyl mercaptan without putting any dead animal underneath it. Turkey vultures still arrived. In his 1964 paper, Stager described birds approaching the odor from downwind and circling the source even when there was nothing there to see or eat.
The anatomy turned out to match the behavior. A modern comparison of turkey and black vulture brains found that turkey vultures have olfactory bulbs about four times larger and roughly twice as many mitral cells, despite having slightly smaller brains overall. Among the birds examined, the turkey vulture had the largest olfactory bulbs both absolutely and relative to brain size.
That equipment is particularly useful in forests, where a dead animal can disappear completely beneath leaves and canopy. In Panama, David Houston tested turkey vultures with chicken carcasses placed in tropical forest. The birds found 71 of 74 within three days, and concealed carcasses were found just as quickly as visible ones. Fresh carcasses were harder. Once decomposition had progressed for about a day and stronger odors were being released, the birds became remarkably efficient at finding them.
Black vultures handle the same problem differently. They do have a sense of smell, and newer experiments have shown they can use it at close range, so the old claim that they simply cannot smell carrion is too strong. But their olfactory system is far less developed than that of turkey vultures. In the wild they rely much more heavily on sight and social information, including watching where turkey vultures descend. They often arrive later in greater numbers and can then push the turkey vultures away from the carcass the other species located first.
Union Oil had no need to know any of that in 1938. Its engineers had a leaking gas line running through rough California country and a sulfur compound that turkey vultures happened to find interesting. They put more of it into the pipeline and watched the sky.
At several places along the 42-mile route, vultures gathered. At those same places, workers found the leaks.
Image is for illustration purposes only
Source: Cornell Lab of Ornithology / BirdNote / Kenneth Stager, Los Angeles County Museum / Union Oil Company of California, 1938 / Popular Science / ScienceDirect.
