Caged canaries where used in the mining. Mining workers carry the birds with them into the mining tunnels. If dangerous gases such as methane or carbon monoxide leaked into the mine, the gases kill the canary before killing the miners. So a dead bird was a warning signal to exit the tunnels immediately. Today, the practice of using a bird to test the air supply has become part of coal mining lore, but the ideology behind it has become a popular expression. The phrase 'living like a canary in a coal mine' often refers to serving as a warning to others. The actual canary had little control over its fate, but it continued to sing anyway. In one sense, living this way indicates a willingness to experience life's dangers without compromise.