The conventional way to detect the absence (or litle quantity of) oxygen is a candle, instead of a canary. This avoids the animal sacrifice. However, I think the candle wastes more oxygen than the canary.

I am not an expert in gas detection, but I can imagine that measuring CO2 is not the same as measuring the absence of O2.

BTW: How many food and oxygen does a middle-sized canary drain from the "power supply"?. Does it drain less current in the sleeping mode?. What peripherals remain active in this mode?. :-)

Alfredo.