Certainly never would I suggest suffocation -- but by
asphyxiation. Hugely different ... there is absolutely no pain, as the bird simply goes to sleep. There is no struggle, no effort to get air, as they system quietly shuts down. Nothing more than the involuntary twitching that occurs when life has ended.
I am no fan of the PETA organization, in that they've pushed the pendulum entirely too far, and it's intended purpose into the past. However, even they are encouraging the use of this method over any other. I have seen no credible information from any reliable source to contradict the belief that asphyxiation is the most humane method. In fact, the only resistance I've seen is by the processing facilities through the PR/lobbying efforts of the National Chicken Council (NCC), which is sorta like asking R. J. Reynolds International if smoking is good for the children they sell their cigarettes to.
Carbon Dioxide at concentrations of 7.5% is an analgesic, and at 30%-40% it can be used as an anesthetic, causing rapid loss of consciousness without struggling, distress, or excitation.
1 At higher than 80% CO
2 causes quick death. It is important to first induce an analgesic effect, then bring about deep anesthesia within a minute or two, and before exposing the animal to high concentrations, so as to cause absolutely no discomfort.
1 when using vinegar and baking soda to produce the required CO2, a small amount of vinegar could be placed w/in the container so they may become accustomed to the odor, and the 'fizzing' action makes a noise that might frighten some animals, but they would otherwise be entirely unaware of this process.