In today's day and age, the canary in the coal mine is used to refer to the person who acts as an early warning sign for others, but the term's true origin is much more direct than that. Where did this phrase come from?
The first use of this idiom dates back to the days of mining when caves had no ventilation systems. During digging, dangerous gases like carbon monoxide and methane could be released into the mines where people would be working, killing them before it could be detected.
To combat this, miners in America and the U.K. began to bring a caged canary with them to the mine, a bird that is more sensitive to the toxic fumes than humans. As long as the canary was singing, the area was safe, but if gases were to be released, the canary would succumb to the poison early on, signaling a gas leak for the miners so they could get out safely.
This method was used into the 20th century until mining companies adopted better detection technologies, an innovation that the little birds certainly welcomed, allowing them to keep on singing.