canary in a coal mine
idiomSomething 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.
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canary fit
A highly emotional state of distress, anger, etc.
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cat that ate the canary
A person who appears self-satisfied or smug, especially while concealing something mischievous, prohibited, or private.
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miners' canary
Any thing, especially an organism, whose demise or distress provides an early warning of danger.