That's really cool. I had assumed they always sacrificed the bird and I can't imagine everyone being OK with that over time. Regardless of the cost, most people don't relish the idea of killing an animal even if it means their own survival so it doesn't surprise me that an invention like this exists.

As the article states, I can imagine the miners responsible for this duty became attached to the birds (especially after they had a way to keep it alive).

There's a detail that got lost in the shuffle there, though. While it had been possible to detect CO presence since -- I think -- the 20s or 30s, they didn't become common until the 80s. My Dad used to fly a private plane across the country selling CO detection solutions provided by his company to factories and businesses until the late 80s (from a little company in Ann Arbor, MI). Of course, come the mid-90s, most of us had devices that could detect presence and (to a lesser degree) levels in our homes.