At one time, not so long ago, there were actual canaries in the coal mines. The origins of the metaphor lie in the work practices of coal miners that would bring caged canaries into the coal mines with them. Throughout the early and into the 20th century, canaries were used to warn miners of toxic gases such as carbon monoxide in coal mines in what is now known as Canada as well as in Britain and other European countries. Being more susceptible to the gases, the canaries would collapse and die. They were often female canaries because they were not as valued.
This cage (circa early 20th century, exact date unknown) was a coal miner's canary cage that would have been brought into a mine. Today, the westslope cutthroat trout are referred to as 'a canary in the coal mine' as the fish population continues to decline as a result of habitat loss and destruction caused by resource extraction, climate change, and other human impacts.
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