All birds have uniflow lungs and a syrinx. What bothers me is that what we were taught in school biology turns out to be stories for children. We were told that erythrocytes (red blood corpuscles) don't have nuclei (true - I've seen them), and this is so they can fold up small to get through tiny capillaries and get oxygen right into muscles. But birds (and reptiles) do have nucleated erythrocytes. I've not noticed that birds' muscles are noticeably less powerful or have worse endurance than mammals. Not many mammals can fly, but most birds can; swallows fly to Africa and back every year.