"Stager was told how in the 1930s [the Union Oil Company of California] had noticed that leaks in natural gas pipelines attracted turkey vultures. The gas contained ethyl mercaptan (aka ethanethiol), a substance that smells like rotten cabbage (also responsible for the smell of bad breath and flatus); it is also released from decaying organic matter, including animal bodies. Union Oil therefore added higher concentrations of mercaptan to the gas to help them locate leaks."

from Bird Sense by Tim Birkhead