The Evolution of Flight.... why are some birds grounded?

One point from Andya's post struck me as interesting, although not directly in the form posted. We would reasonably expect flight, or indeed gliding as a pre-cursor, to require a strong, and flexible, muscle structure in the (to-be-)flight limbs. These pre adaptions are, surely, more likely to develop in an arboreal creature than a bipedeal runner.Since you seem to have a pretty solid grounding in this area, I'm wondering whether you think this is a worthwhile line of argument?

Mark24,Thanks for your reply, that answers my question nicely. I must take issue with this. The forelimbs of humans are massively less powerful than those of other apes. Most other apes are easily capable of pulling themselves up on one hand without apparent effort - a feat that is exceptionally rare in modern humans. Even top-class climbers can't match the ease with which large apes such as Orang-utans can pull themselves around using just their upper body strength.