Canary In A Coal Mine Meaning

(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.

Example: 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 9781401917555, 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.