pruner
How about a small tree pruner? A less messy option would be to carry a few rubber bands and loop one around its neck a couple times, would be dead quite quickly. How about a 300000v taser like you use for self defense? I know they can knock a dog down for a good minute, small animals would probably react worse, might even kill them. You could try sticking a firecracker up its ??? and lighting it? Better yet use a whistleing bottlerocket. How about stepping on its head till you hear some crunching? You could throw it up in the air and shoot it again on its way down. Or run it over with your truck. Stick its head in the exhaust pipe of your atv or truck and rev the engine for a minute or so. Find a body of water and drown it. Im sure there are more ways, but I dont have the time. Really its not hard to kill a bird thats already been shot, they are fairly fragile animals.