Wallace Berry wrote:

The bagpipe is a good analogy, with the bird's airsacs as the bags, and
the drones and chanter of the pipe analogous to the lungs. Birds lungs
are rigid and don't expand and contract the way mammalian lungs do. All
the bellows work is done by the airsacs.

But all this doesn't solve the main problem: low partial O2 pressure.

Stefan