Canary in the Gold Mine
The Domestic Canary, often simply known as the canary, is a domesticated form of the wild Canary, a small songbird in the finch family originating from the Macaronesian Islands. - Wikipedia
"Canary in a coal mine" is an often used metaphor.
The source image for this caricature of a canary is a Creative Commons licensed photo from David Carroll's Flickr photostream.