Evolution of bird lungs from reptile lungs impossible?

"Initially birds evolve to breath by expanding/contracting the air sacs rather than the lungs: this is beneficial because it frees the cycle of breathing from the beat pattern of the winds."I see a major problem... micromutions resulting in the ability of the alveoli to exand and contract would not have a selective advantage over functional lungs... yes, it would free the cycle of breathing from the beat pattern of the wings, but it would result in less oxygen being utilized. As in other examples in nature, transitional forms would be less functional and thus would not survive. Your analogy is thus untenable.