miner's canary

Something or someone who, due to sensitivity to his, her, or its surroundings, acts as an indicator and early warning of possible adverse conditions or danger. Refers to the former practice of taking caged canaries into coal mines—the birds would die if methane gas was present, thereby alerting miners to the danger. Wildlife in disaster movies always acts like the miner's canary, fleeing the scene when catastrophe is imminent. John was used as a miner's canary to see the test drug's effects on the human mind.