I'm genuinely curious now.
If you have 440 squirrels to kill, in an airport, that are legally not allowed to leave the airport... how the heck would you do it humanely?
It feels extremely non-obvious to me. Are there extra-large portable CO2 chambers the size of a whole room or something you can rent? And whose budget is that coming out of? I'm assuming nobody's going to try to inject them all individually with something.
Getting suffocated with CO2 is also a terrible way to die. Animal welfare research is clear on that. There are other options (with other gases that just make the animal fall asleep and avoids the suffocation reflex) but they are still more costly currently. If we'd switch to those then one reason I feel uncomfortable as a meat eater would disappear.