actually the gas pipe technique is pretty humane, and much easier psychologicaly. Every year there are thousands of accidental human suicides from people whose tailpipes are blocked up without them knowing it, if there were painful pollutants they would probably realize that they couldn't breath and get out of the car, instead they slowly fall asleep. Sorry to be so, creepy, i guess. Another method that works well is engine starter fluid, put the bird in a ziplock bag, put a paper towel over it, spray plenty of starter fluid in and seal the bag, the bird is gone in like 10 seconds, plus they get wikid high, like the idiots who use it as an inhalant drug, so the bird dies happy. I used to use this method all the time, i'm not 100% on the details so do some research, but if i were you this is what i'd do. Cutting the head off will be quick and painless, but only for the bird, i can remember when i took out my first bird like this, i couldn't get the image out of my head for weeks... (i was like 14, give me a break)