canary in the coal mine

canary in the coal mine (plural canaries in the coal mine or canaries in coal mines or canaries in the coal mines)

1. Alternative form of canary in a coal mine
   • 2006, Al Gore, An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It, Rodale, →ISBN, page 176:

     The second canary in the coal mine — along with the Arctic— is Antarctica, the largest mass of ice on the planet by far.

   • 2007, Kentwood D. Wells, The Ecology and Behavior of Amphibians, The University of Chicago Press, →ISBN, pages 784–785:

     Many authors state, almost as a truism, that amphibians are more vulnerable to environmental insults than are other vertebrates and therefore might serve as a kind of “canary in the coal mine,” an indicator of environmental degradation (e.g., Carroll 1999).

   • 2009, Erica Manfred, He's History, You're Not: Surviving Divorce After 40, GPP Life, →ISBN, page 167:

     Sex is the canary in the coal mine. It's the first thing to go when a marriage is in trouble.