I just found out that birds breathe very differently from mammals.
You know how mammals breathe: they breathe just like you and me. The lungs expand drawing in air, then compress to exhale, the air flow reversing direction. In and out.
Birds have lungs, and also numerous air sacs. The lungs don’t change size much. It is the air sacs that do the work, and they blow the air through the lungs. Through, not in and out. Valves ensure that the air flow is one-directional.
The lung is the small rectangular bit on top. The large air sacs do all the work.
I hadn’t really appreciated how different birds are from mammals. Sure they have feathers. But some mammals have scales! But this breathing system of birds is a very different design!