It's a reference to the canaries that were used in coal mines to detect carbon monoxide. Since then 'canary' has been used to refer to early warning systems. h

It's a reference to the canaries that were used in coal mines to detect carbon monoxide. Since then 'canary' has been used to refer to early warning systems. https://en.wikipedia.org/wiki/Sentinel_species https://en.wikipedia.org/wiki/Sentinel_species