[Tax-chick]

"We could see the cave's lethal properties during the excavation," he said. "Several birds died as they tried to get close to the warm opening, instantly killed by the carbon dioxide fumes."

Not carbon dioxide! (Do birds die if people exhale on them?)

[Oberon]

Not carbon dioxide! (Do birds die if people exhale on them?)

Get yourself surrounded by pure carbon dioxide, and you will asphyxiate PDQ.

(What we breathe out is only a percent or two richer in carbon dioxide that what we breathed in.)