What drove bird evolution?

Of course having read that they might notice that they only have a 'flow through heart' in the same way that many other vertebrates do, i.e. one which blood flows/is pumped through, so perhaps you should have educated yourself similarly before you posted. The lungs are an interesting feature but not neccessarily required for flight, see bats once again for a counterexample.TTFN,WK

And yet they do fly, so flight itself doesn't require these elements. The flight of modern birds may require them, but this poses no problem for them being absent in their present forms in an ancestral bird which may also not have flown as high, fast or far as modern birds.This seems to be pretty much baseless assertion, do you have any evidence that all of these things are absoloute prerequisites for flight and have no use in a non-flight context. Beaks seem a particularly strange as a choice given, as has already been mentioned, the number of non birds which have them.TTFN,WK

Its a beluga whale embryo, naturally.

It is just as well then that this research was not based on fossil evidence. The paper is based on phylogenies derived from molecular data.Admittedly this appears to be a case of 're-evolving' wings only in as much as it probably represents the loss of one or two key genes in the pathway which were then functionally recapitulated not the de novo re-evolution of the entire system, the rest of the developmental mechanism haviong been maintained during the gap due to their roles in other organs, i.e. limb development.TTFN,WK