Canaries were used in coal mines to detect the presence of carbon monoxide; if the canary is dead, you will soon be.
A fairly old trope; not sure if we already have this one. Quoth the Wikipedia: "Canaries were iconically used in coal mines to detect the presence of carbon monoxide. The bird's rapid breathing rate, small size, and high metabolism, compared to the miners, led birds in dangerous mines to succumb before the miners, thereby giving them time to take action." Shows up in historical works (or period pieces, or some fantasy novels) about mining, naturally. Nowadays, if it still makes an appearance, it's generally done metaphorically (such as mentioning that a character with a danger sense "is like a canary in a coal mine" in that, when they panic, it's time to run away). Western Animation
- In the "Joker Express" episode of The Batman, the Joker uses hypnotic lights in the rail-system to have other people steal for him. They then dump it in the river and he collects them. Batman, Robin and Batgirl track him down to an old mine. Here Joker is seen wearing a classic Railroad engineer outfit, complete with a small, handheld cage with a canary skeleton in it.
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