canary in a coal mine

• canary in the coal mine

An allusion to caged canaries (birds) that miners would carry down into the mine tunnels with them. If dangerous gases such as carbon monoxide collected in the mine, the gases would kill the canary before killing the miners, thus providing a warning to exit the tunnels immediately.

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canary in a coal mine (plural canaries in a coal mine or canaries in coal mines)

1. (idiomatic) Something whose sensitivity to adverse conditions makes it a useful early indicator of such conditions; something which warns of the coming of greater danger or trouble by a deterioration in its health or welfare.
   • 2003, "Saving the St. Joe", Field & Stream, April 2003:

     "A mussel is a canary in a coal mine," Clemens explained. "When the freshwater mussels are healthy, it indicates good water quality."

   • 2008, Jean Haner, The Wisdom of Your Face: Change Your Life With Chinese Face Reading!, Hay House (2008), →ISBN, page 133:

     On the job, she was like a canary in a coal mine: If the subtle energy wasn't in balance, she was the first to notice.

   • 2009, Steve Kandell, "What Makes U2 Run?", Spin, April 2009:

     Thanks to rampant real estate development that revitalized the city starting in the '90s, Dublin has been something of a canary in a coal mine with regard to the global financial meltdown — the credit crunch hit here early and hard.

• miner's canary

• warrant canary