Its the reason natural gas smells like it doez. If there is a leak they can look for birds in the area.
Not to split hairs with you, but I worked in the field the last tweny years before retiring, and when we looked for a leak, we looked for dead vegitation over the leak and used "sniffers".
Natural gas has no oder naturally, they add Mercaptain to it to give it the rotten egg smell it has. Most major pipelines have none added till it is branched off for use.
Never once in twenty years did I ever hear of birds used to find a gas leak!