canary in a coal mine

plural canaries in a coal mine · plural canaries in coal mines · alternative canary in a coalmine · alternative canary in the coal mine · alternative canary in the coalmine

1. idiomaticSomething whose sensitivity to adverse conditions makes it a useful early indicator of such conditions; something which warns of the coming of a greater danger or trouble by a deterioration in its health or welfare.

   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.

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

   2 more examples

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

   2003 April, “Saving the St. Joe”, in Field & Stream

   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.

   2009 April, Steve Kandell, “What Makes U2 Run?”, in Spin

   Synonyms: miner's canary