Are flightless birds a reversion?

no.i'm not the best person to ask, really, because i'm one of those people that holds the attitube of "just what the hell is a bird, anyways?" all birds, however, seem to decended from a single flying ancestor, something very much like archaeopteryx.it's interesting to note that flight seems to have, at the very least, been lost repeatedly by birds. possibly re-evolved. it's even quite possible that some dinosaurs we know and love (like velociraptor) evolved from ancestors that flew (archaeopteryx seems to be relatively close to the ancestral deinonychosaur). essentially modern birds were living in the cretaceous, and soon after the death of the dinosaurs, and until just two million years ago, we had giant terror birds running around filling essentially dinosaurian niches.oh, and i was going to make a separate thread about this later -- did you hear we've got another example of a cenozoic dinosaur? apparently, dinosaurs only died about sixty-FOUR million years ago, and seem to have survived the asteroid impact by about a million years.

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this might help:
(source)

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because the development of a carpometacarpus so wonderfully recapitulates the evolutionary history of the bird, it's actually totally possible for some birds to simply re-develop claws and such thought neotonoy.i'm not sure about ratite claws. i've heard mixed information, and i'm no ornithologist. well, very odd things DID happen. the question is whether this is a plausibility. the answer in this case, however, is "no."

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i find it interesting that flightlessness is actually such an advantage that birds keep secondarily loosing flight.

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well, that can't be entirely true. we never seem to see, for instance, warm blooded animals "reverting" to cold blooded. at least, none that i am personally aware of. yet, cold bloodedness is VASTLY superior in terms of energy usage. cold blooded animals simply do not need to catch and eat as much food, because they do not have to worry about keeping up body temperature with their metabolism.this is probably because no one particular advantage is weighed in a vacuum. it is the combination of factors and how they interact that matters most.

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okay, that's just really cool.