2006, Anthony Marcus, Where Have All the Homeless Gone?: The Making and Unmaking of a Crisis, Berghahn Books (2006), ISBN 1845450507, page 152:
The weakest members of society become social barometers or canaries in a coal mine.
2006, Steve Fennessy, "Frog Man", Atlanta Magazine, July 2006:
One of the few animals to live part of its life in water and part on land, frogs are seen by some as canaries in a coal mine — which is why the chytrid fungus has environmentalists worried.
2009, Patricia K. Smith, Obesity Among Poor Americans: Is Public Assistance the Problem?, Vanderbilt University Press (2009), ISBN 9780826516350, page 13:
Perhaps public assistance has little to do with obesity among the poor; instead, the poor may simply be canaries in a coal mine, their health suffering from environmental changes before others feel the impact.