Canary in a Coal Mine

Life for an actual canary in a coal mine could be described in three words - short but meaningful. Early coal mines did not feature ventilation systems, so miners would routinely bring a caged canary into new coal seams. Canaries are especially sensitive to methane and carbon monoxide, which made them ideal for detecting any dangerous gas build-ups. As long as the canary in a coal mine kept singing, the miners knew their air supply was safe. A dead canary in a coal mine signalled an immediate evacuation.

Even as gas detection technology improved, some mining companies still relied on the 'canary in a coal mine' method well into the 20th century. Other animals were used occasionally, but only the canary had the ability to detect small concentrations of gas and react instinctively.
Today, the practice of using a canary in a coal mine 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 in a coal mine had little control over its fate, but it continued to sing anyway. In one sense, living like a canary in a coal mine indicates a willingness to experience life's dangers without compromise

Source: wisegeek.com