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 as male canaries.
This device was used to resuscitate canaries. When a canary began to exhibit signs of distress from carbon monoxide poisoning, they would be placed inside. A valve would be opened, and oxygen would fill the chamber, with the aim of reviving the canary.
From the Royal Alberta Museum:
Object Name: bird cage, Mine Testing
Description: mine type bird cage. has a square chrome rim on each side which holds in the mica window. hinged, oval-shaped front door with chrome rim & mica window, with swivel wing nut for closing & tightening the door. has a tubular-shaped metal oxygen bottle on top with valve handle for regulating flow, clamped on by brass "c" clamps. has a small release valve for pressure release. has a wooden perch inside.
Thank you to the Royal Alberta Museum for the loan of this object.