Canary in a Coal Mine
"Canaries were once regularly used in coal mining as an early warning system. Toxic gases such as carbon monoxide and methane in the mine would kill the bird before affecting the miners. Because canaries tend to sing much of the time, they would stop singing prior to succumbing to the gas therefore alerting miners to the danger." (Wikipedia) Around the former coal mine "Zollverein" in Essen you can find 10 or 12 larger than life canaries to remind on this miner's practice.