canary in the coal mine

noun phrase

: the first sign of trouble : something serving as an early warning or indication

Measles is very contagious, and so the wastewater itself is just our canary in the coal mine, as it were, telling us that measles is in the community.

—Christine Hahn

The sneaker resale market's struggles may be a canary in the coal mine for broader economic concerns.

—Stacy Fernandez

Crocuses, being more sensitive than trees or shrubs, act as canaries in the coal mine for spring shifts.

—Lionel Smith

Word History

Etymology

from the former practice of keeping a caged canary in a coal mine on the assumption that the bird would be affected early by carbon monoxide or other toxic gases

First Known Use

1920, in the meaning defined above