canary in a 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.
- (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 April, “Saving the St. Joe”, in Field & Stream:
- "A mussel is a canary in a coal mine," Clemens explained. "When the freshwater mussels are healthy, it indicates good water quality."
- 2009 April, Steve Kandell, “What Makes U2 Run?”, in Spin:
- 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.