There is a book that examines the possible origins of these creatures, it’s called Out of Thin Air - Dinosaurs, Birds, and Earth’s Ancient Atmosphere - by Peter D. Ward (2006; Joseph Henry Press).
Mr.Ward speculates on an oxygen crash as the cause of the Permian extinction and how dinosaurs had a respiratory system that differed from other creatures in that they could hoard oxygen in ‘bladders’ throughout their bodies.
Climbers on Mt.Everest have been surprised to discover upon reaching the summit, flocks of birds crossing over the mountain above them. Since the effort to fly uses huge amounts of oxygen, how can birds do that?
This book tries to answer the question.