That was kind of the point of my definition, actually—it's easier to predict what type of thing is a bird if you think of "bird" as "the clade rooted on all the theropods that fit through a particular evolutionary sieve" rather than a category with certain characteristics. Not all modern birds have those characteristics. But those characteristics are what allowed the prototypal "birds" to survive where other dinosaurs did not. Their descendants went on to adapt in various other ways, but we still consider them all "birds" for reasons of genetic lineage back to those original birds.

Sorry, I misunderstood you and thought your "flight" point was meant to be valid for current species as well as the early incarnations. Appologies.

Yes, they evolved later (generally under non-mammalian island biogeographies which produced evolutionary pressures which meant that being large and foraging nocturnally had benefit over retaining flight).

This happens all the time -- imagine the evolutionary pressures on proto-cetaceans. Now imagine writing a comment discounting the benefits of land-dwelling because some paths returned to the sea.

Indeed. However where I erred was misunderstanding the GP. I thought his "flight" description was intended to be relevant to present birds as well.

Flightless birds are birds that lost their capability to fly.