Birds and dinosaurs share a respiratory system that circulates fresh air constantly moving in the same direction, whereas mammals breathe by inhaling and exhaling. Of the two, the constant circulation method is more efficient because air is completely expelled from birds lungs but not from mammalian lungs which always have a reservoir of un-expelled oxygen depleted air that’s high in CO2. Over millions of years mammals evolved a higher tolerance for CO2 than birds (or Dinosaurs). As evidence, I submit that birds don’t keep caged people as indicators of bad air in their coal mines, probably for that reason.
"A small fraction of the Cretaceous bird species survived the impact, giving rise to today's birds. So far, only a single bird species, which has not been named, has been confidently identified from both above and below the K–Pg boundary (it is present in the Maastrichtian Hell Creek Formation and Danian Fort Union Formation). The only bird group known for certain to have survived the K–Pg boundary is the Neornithines (though one Paleogene species, Qinornis paleocenica, may represent a surviving non-neornithine bird) Neornithines may have been able to survive the extinction as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of neornithines can build burrows, or nest in tree holes or termite nests, all of which provided shelter from the environmental effects at the K–Pg boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs." (Wikipedia)The major survivors of the Cretaceous extinction appear to have been small, subterranean dwellers where, like Naked Mole Rats, they had an even higher tolerance to CO2 than usual. Flight doesn't seem to have been a survival trait in this case.
Many surviving Reptile and Crocodile species also nests in burrows, it’s assumed that extinct species did too. Turtles are adept at living in low oxygen environments (underground and underwater).
A theoretical test might be to raise some birds in a high (but not fatal) CO2 environment to see how their breeding capacity fares over time. It's my hypothesis that restricted oxygen will affect foraging ability, thus brood size. High School Nerds: If you want that science fair prize, this is your chance (& let me know what happens). I suggest Button Quail, vs mice because both are cheap, Canaries and Naked Mole rats are not. Some plan should be employed so their effort to access food is adjustable so as to be measurable in relative terms, and so calories might be estimated. Possibly by placing the feeder so they have to work to get to it but can't hang out there to rest.