Yellow Canary in a Coal Mine Best Friend

Sentinel species are organisms, often animals, used to detect risks to humans by providing advance warning of a danger. The terms primarily apply in the context of environmental hazards rather than those from other sources. Some animals can act as sentinels because they may be more susceptible or have greater exposure to a particular hazard than humans in the same environment. People have long observed animals for signs of impending hazards or evidence of environmental threats. Plants and other living organisms have also been used for these purposes.

The classic example is the "canary in the coal mine".