When they kill rodents for the reptile food industry they use dry ice. Just put them in a bog box with a piece of dry ice in a small tub and pour some water in the tub, then close the box. The dry ice releases lots of carbon dioxide and displaces the oxygen. From what I understand it happens...
I know this is not the answer you are looking for, but I find the most humane way to put a chicken down (not slaughter for eating) is a very carefully placed shot from a 22 pistol to the back of the head. if you destroy / sever the base of the spinal column / motor cortex / Modula Oblongata all...