Birds have the most highly efficient respiratory system amongst the terrestrial animal kingdom. Air passes through their lungs twice, during the inhalation AND the exhalation. That’s where gas exchange (oxygen and carbon dioxide) takes place. The excess air that doesn’t fit in their lungs is held in their 9 (!!!) air sacs.
Show-offs.
And, like, this is totally unnecessary.
Bats prove this. They’re mammals like we are, and they are able to traverse migratory distances using the same simple air-in-and-out lungs that we have.
One theory for why birds have such a complex system is that proto-dinosaurs evolved that adaptation back during one of those major extinction events when oxygen levels dropped and killed 96% of all marine life. So this was just an evolutionary vestige that later turned out to be super useful for creatures that fly long distances. (Bird feathers similarly seem to have had a different original purpose - thermal insulation, and courtship. But turns out feathers also helped with gliding, and eventually flight.)
Birds emerged from a complicated world and then utilized the gifts they acquired there to adapt to a new one. We could all strive to similarly transmute ourselves to achieve new capacities.
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