Animals Series. Episode #3 of 4. The term “canary in a coal mine” is ubiquitous for any early warning signal. Like our fictional vignette of a miner carrying a canary into the coal mine, canaries were often taken into mines during the first part of the 20th century to test the air for poisonous gasses. The practice was so commonplace that it's become a cliché. Metaphors aside, canaries are a sentinel species, used by humans to detect environmental risks by providing advance warning of a danger. Often animals are used as sentinels because they are more susceptible to environmental hazards that humans may be in the same environment. In the case of coal mining, canaries -- or really any small bird -- are very susceptible to changes in air quality because of their rate of respiration, anatomy, and small size. Contrary to popular belief, canaries in coal mines do not have a very long history. They were only used as sentinel animals in British and American coal mines for roughly 100 years. In the grand scheme of things, that’s not a long time at all. Yet, canaries have become ubiquitous with mining in general and as a figure of speech.

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