Canaries were once regularly used in coal mining as an early warning system. Toxic gases such as carbon monoxide, methane or carbon dioxide in the mine would kill the bird before affecting the miners. Signs of distress from the bird indicated to the miners that conditions were unsafe. The use of miners' canaries in British mines was phased out in 1987.
The phrase "canary in a coal mine" is frequently used to refer to a person or thing which serves as an early warning of a coming crisis. By analogy, the term climate canary is used to refer to a species that is affected by an environmental danger prior to other species, thus serving as an early warning system for the other species with regard to the danger. (See indicator species.)