The saying ‘a canary in a coal mine’ is often used to describe something that warns of looming trouble by it’s reaction to adverse conditions. Coal miners would carry caged canaries into the mines and if there was a poisonous gas leak, the canaries would die before it affect the miners. The canaries sensitivity to the condition was a warning of danger.
The article attached suggests that commodities are the canary in the coal mine “for equity valuations, with big divergences sometimes signalling major price events just ahead…” (Keenan, 2014).
Read more here: Are commodities pointing to better days ahead?