Wiktionary
canary in a coal mine
n. (context idiomatic English) 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.
Usage examples of "canary in a coal mine".
Like a canary in a coal mine, _The Purple Cloud_ warns us that the anxieties of living on a planet have just begun.
Keeping Dmitri with me as a test - like a caged canary in a coal mine - I went into the laundry room and inspected the circuit breakers.
Bad luck that Rebecca had been playing with her rabbit at that exact moment, which acted like a canary in a coal mine and warned the doctor in time.