A reference to caged canaries that miners would carry down into the mine tunnels with them. If dangerous gases such as carbon monoxide collected in the mine, the gases would kill the canary before killing the miners, thus providing a warning to exit the tunnels immediately. “A canary in a coal mine” is also a metaphor for something whose sensitivity to adverse conditions makes it a useful early indicator (such as blood sugar health) something which provides a warning for a coming greater danger (such as a deterioration in health leading to a chronic metabolic disease such as type II diabetes).