What Is A "Miner's Canary"?
"Miner's canary" refers to the birds once used by miners to test the purity of the air in the mines. At least three birds would be taken underground by a group of miners. If any one bird showed signs of distress, it was taken as an indication that dangerously high levels of carbon monoxide existed. Some miners used mice rather than birds. Today, miners use sophisticated sensing equipment instead of canaries.
Sources: Thrush, Paul W. A Dictionary of Mining, Mineral, and Related Terms, p. 168; U.S. Bureau of Mines. Bulletin No. 42,1913,p. 66.