Wiktionary
miners' canary
alt. 1 A caged bird kept caged in mines because its demise provided a warning of dangerous levels of toxic gases. 2 (context idiomatic English) Any thing, especially an organism, whose demise or distress provides an early warning of danger. n. 1 A caged bird kept caged in mines because its demise provided a warning of dangerous levels of toxic gases. 2 (context idiomatic English) Any thing, especially an organism, whose demise or distress provides an early warning of danger.
Usage examples of "miners' canary".
She's right, it's one of the tattletales - losing twenty pounds just like that is like when the miners' canary drops dead in his cage.