Canary in the Coal Mine

Etymology

An allusion to caged canaries (birds) that mining workers would carry down into the mine tunnels with them. If dangerous gases such as methane or carbon monoxide leaked into the mine, the gases would kill the canary before killing the miners, thus providing a warning to exit the tunnels immediately.

1. (idiomatic) Something whose sensitivity to adverse conditions makes it a useful early indicator of such conditions; something which warns of the coming of greater danger or trouble by a deterioration in its health or welfare.

-Wikipedia