When exposed to toxic gases, canaries suffer ill effects and die sooner than humans do. That’s why coalminers used to bring caged canaries into the mines with them. If the canaries became sick or died, this was a sign that something was seriously amiss and that the miners needed to get out.
The practice was phased out, at least in the U.S. and the U.K., by the late 20th century, but the phrase canary in the coal mine lives as a metaphor for any warning of serious danger to come. The canary is not prophetic until it is brought in the coalmine, so the metaphor works especially well if the prophetic thing is small, innocent, and not prophetic under normal circumstances. Few writers take pains to make the metaphor perfectly logical, though, and most so-called canaries in coalmines are just things that should be considered alarming.
The current confrontation over contraceptives, sterilizations, and abortifacients is thus a rather large canary in the religious coal mine. [American Thinker]
The stories of senior citizens in distressed economic conditions because of defaulted student loans are merely the canary in the coal mine. [Washington Post]
Like canaries in the coalmines of yore, low-lying islands in the midst of the world’s vast oceans face the possibility of extinction. [The Atlantic]
Every now and then the canary in the coal mine does indeed die — it would be kind of foolish to blithely wave away this particular dead bird just because it might signify a freak avian heart attack. [Salon]
It should have been a canary in the mine that for every tweet in favour, there were lots more first preferences being inked on to ballot papers. [BBC]
If your canary in the mineshaft sprouts an extra head, you might think there’s a problem. [National Geographic]