Yuck, horrible topic. So far Archcat has mostly brought in live, uninjured prey which we've just caught and released, or he's killed it himself. But the mouse two days ago was clearly in a bad way, with a badly injured leg, hardly able to move, probably in shock, and Archcat had lost all interest and wandered out again. I'd already read up the best way to humanely euthanise because I suspected we'd have to do this some time: apparently you put the animal in a sealed small box with a bowl containing a baking powder and vinegar mix, which releases enough carbon dioxide to kill the mouse or bird gently and painlessly. It actually worked, we checked half an hour later and the mouse was clearly properly dead by then. Anyone else done this before? Are there any more humane options?