Chickens aren't humans. The CO2 asphyxiation response seems to very across species. For example, this agricultural guide claims that chickens lose consciousness

Chickens aren't humans. The CO2 asphyxiation response seems to very across species. For example, this agricultural guide claims that chickens lose consciousness before reaching the response phases that we would consider painful: > While rats do not lose consciousness prior to aversive responses to CO2, chickens become unconscious very quickly in high concentrations of CO2 and somatosensory neuronal potentials are not evoked until several seconds after the birds are unconscious. > ... > Webster and Fletcher (2004) reported that while White Leghorn chickens could detect atmospheres enriched with CO2 at 30, 45, and 60% in air, there were no differences in behaviors associated with the various atmospheres. They concluded that most hens, given modest motivation, were not sufficiently deterred from entering any of the CO2-enriched atmospheres. ... Source: https://animalcare.illinois.edu/standards/co2-euthanasia-poultry-and-young-swine-guidelines https://animalcare.illinois.edu/standards/co2-euthanasia-pou...