"To act like the canary in the coal mine" is an idiomatic expression referring to the literal sense and situation of the canaries that were used in coald mines. As I understand, canaries were used, in the 19th C and early 20th C, in coal mines, not as pets or to keep company to the miners, but as zoological early-warning systems for toxic gases or fumes. Canaries being tiny birds would choke and die earlier than a man would. In other words, when the canary was off-colour, all hands knew that trouble was brewing and that they should take action (i.e. escape). But I would like details as to how the metaphorical expression is used, i.e. what is implied precisely. Is it merely to act as whistle-blower? Is there the idea that one is being misused or abused? (The canaries did not ask to be stuck underground like that.) Is it positive (about the person seen as a canary) or rather negative (since he or she may be deemed a bit of a fool)? Is there any reference to the colour? (At first, I thought it was the idea of "the black sheep" since, supposedly, a yellow canary in a dark black coal mine would stand out, somehow.) Insight welcome and Xmas greetings from me.