Much like their dinosaur ancestors, today’s birds have nine different air sacs that function like bellows. They breathe into their air sacs and out through their lungs. That means they have way more oxygen available at any given moment than mammals do, so it’s a lot easier for them to do ridiculously energetic things like flying.[*4] And singing all day long. Singing is, in many respects, a fancy way of holding your breath, much like talking.
