[thedoc (OP)]

Doug asked the Naked Scientists:
   Hi.  You recently referred to the fact that birds have a different type of lung to reptiles, mammals, etc.  Air moves through bird lungs in a continuous non-stop circulatory flow, in contrast to the in-and-out bellows-type lungs of reptiles, mammals, etc.  

According to evolutionary theory, birds evolved from reptiles in a very slow, very gradual manner over a very long period of time, with every successful small step being a functional improvement (an advantage for survival) compared to what came before.

Does science have any idea how a bellows-style reptile lung can change to a bird lung in a very slow, very gradual manner over a very long period of time, with every successful small step being an advantage for survival compared to what came before?

It seems to me that one of the unavoidable intermediate stages in this process is death.
What do you think?