Canary in coal mine
I came across this interesting "usage" as described on the BBC website:
(Picture from the BBC website) | The canary is particularly sensitive to toxic gases such as carbon monoxide which is colourless, odourless and tasteless. This gas could easily form underground during a mine fire or after an explosion. Following a mine fire or explosion, mine rescuers would descend into the mine, carrying a canary in a small wooden or metal cage. Any sign of distress from the canary was a clear signal the conditions underground were unsafe and miners should be evacuated from the pit and the mineshafts made safer. |