Is there a single word to describe the canary in the phrase "Canary in a coalmine"? The best I can come up with is sentinel, but I'm nearly positive there's a more precise option.
Edit: To clarify I'm not looking for words that describe the canary but rather for a word the encapsulates the entire phrase. That is, I don't want to finish the phrase "the canary is a ...". I do want to finish the phrase "$PERSON was a ..." where the blank implies as many of the attributes of the proverbial canary as possible (e.g. going down the dangerous road first, warning those that follow, connotations of captivity, &c). I'm reluctant to use the phrase, or "canary" as it strikes me too colloquial/lighthearted for the work at hand.
@senex suggestion of bellwether and @bib suggestion of tocsin are closest to what I'm looking for thusfar.