The phrase "a canary in a coal mine" refers to the demise of the weakest member as a warning of danger to the rest of the group. The phrase comes from the use of real canaries as carbon monoxide detectors.
Canaries were used in mines from the late 1800s to detect gases, such as carbon monoxide. The gas is deadly to humans – and canaries alike – in large quantities, but canaries are much more sensitive to small amounts of the gas, and so will react more quickly than humans.
This was discovered by John Haldane, who was asked to help determine the cause of an explosion at Tylorstown Colliery in 1896. He concluded the explosion was caused by a build-up of carbon monoxide and set out to find a way of detecting the odourless gas before it could harm humans.
You might assume, as I did, that canaries were considered expendable and often died from the practice. That's not quite so -at least not so in all mines. The cage pictured is a canary resuscitation device, with its own oxygen supply that could be turned on to revive a distressed canary, while men would evacuate the mine. Read how it worked at Museum Crush. -via Nag on the Lake