The thankless task of being the first one to take a hit, from the practice of coal miners of bringing down a caged canary to see if there's dangerousgas below
I don't know if this alleyway's safe, and I ain't no canary in a coal mine.
Office canary: a worthless or virtually worthless employee who will definitely be fired before you, but if that person does get fired, you had better look out.
Very similar to the canarys miners would take into the mine with them to warn them about poison gas. If the canary died, it was time to get out of dodge…