CONNECTICUT FOUNDATION FOR ENVIRONMENTALLY SAFE SCHOOLS

Why "Canaries"

Before more sophisticated equipment was available, miners brought canaries into the mines and mineshafts to to tell if lethal gases such as carbon monoxide and methane were present. The build-up of gases was not otherwise detectable - until it was too late. If the canaries died in the mine shafts, miners did not go down. If the canaries died while they were in a working mine, miner knew they had to get out asap.

More about "yellow canaries"
Center for a Livable Future
1986: Coal mine canaries made redundant