American stockpicker Jim Jubak on the story of the real canary in the coal mine:

“Canary in a coal mine” is an indicator of danger. In the dark-as-a-dungeon days of mining, miners would sometimes carry caged canaries with them into the tunnels. The birds would die from mine gases that the miners themselves hadn’t yet detected. Which was hard on the canaries, of course, but offered the miners a warning to get the hell out of the tunnel in fresher air.

BirdNote explains,

Beginning in 1911, miners in Great Britain carried a canary in a cage with them down into the mines. Why? Carbon monoxide can build to deadly levels, and it has no smell. If the canary weakened or stopped singing, miners knew to get out of the mine—and quickly. Why use a bird instead of, say, a mouse? It all had to do with the birds’ breathing anatomy: canaries get a dose of air when they inhale and when they exhale, thus a double dose of toxic gases. Thankfully, in 1986, more humane electronic warning devices replaced them.