[Simplicitas]

Another feature that make birds really remarkable, if not mistaken, is that they take in oxygen when they inhale AND exhale. Feathers are great, but a creature such as B6 still needs a lot of energy to fly for 10 days straight.

[delecti]

It's more that their respiratory system is kinda circular. When we exhale, there's still a bit of gas in our lungs, and that reduces how efficiently we can extract oxygen. But the oxygen extracting parts of a bird's lungs are more like a heatsink, with air rushing past it in a consistent direction, rather than back and forth.

[KineticLensman]

Bird lungs have separate pipes for inflow and outflow, while mammals use the same pipe for both. Bird lungs are also relatively rigid, and they use separate air sacks to do the pumping

[xeonmc]

So human lungs are piston engines whereas bird lungs are turbojet engines.