the ether will suffocate them. if you ever watch a chick die this way you will turn to chopping the head off. It is the fastest way and you know the chick only has a sec of pain. the lil one i tried with ether suffered for several minutes terribly before i yanked it out and snapped its neck. the ether is a nasty way to die. its not like the movies where you drop in a second. The carbon dioxide way may be a good one, i have not seen a chick die that way. If you want the chick to have the quickest most painless time you chop their heads off. If you are only worried about yourself and want it to be easy for you you drop them in a box or tupperware and walk away as they gasp and flop around trying to suck up as much air as possible that just isnt there.
I was under the impression that ether is an anesthetic and would therefore limit suffering. My chick peeped for maybe 3 or 4 seconds, although it may have been reacting to my putting it into the container. It was then motionless and if it was breathing, I couldn't see it. I would never walk away from an animal suffering or dying, and it did not gasp or flop around -- no discernible motion at all. I only waited longer (with the chick) to be absolutely sure it was gone.
Perhaps it depends on the amount of ether used. I don't know. Nor can I say definitively that it did not suffer. It didn't look like it, but I don't know enough.
Admittedly I was looking for an option that would be easier for me to face than decapitation. And I may use that option in the future. But I would not worry simply about myself and walk away. I don't think anybody here would.