I have used CO2 to dispatch trapped animals, and my buddy, who is the county animal trapper, has 2 installed in the back of his county work truck, a smaller-coon/skunk sized one and a 55g drum for bigger animals.
CO2 works great, just puts em right to sleep and they never wake up.
The only problem I see, and the reason I dont do it that way- CO2 just kills them, but you are then unable to properly bleed an animal out afterwards.
Birds are pretty delicate and really not too hard to kill, grab em by the body and even a quick shake should pop the neck from its socket.
I like to bleed em out, so I lop the head off and squeeze the body like an orange . . .
If your just gonna freeze them for zoo-animal whole food, co2 and freezing is the way to go.