Almost certainly. Ratites–Ostriches, Emus, Cassowaries, Rheas, Kiwis, and the extinct Elephant birds and Moas–have been flightless for a long time, but their ancestors were flyers. They didn’t evolve from a flightless ceolurosaur to a flightless bird.
Of course, there are other flightless birds, like Penguins, or flightless cormorants and so forth, but these have very clearly secondarily lost flight more recently.
Yes. And no it’s not a stupid qusetion. In fact it’s a very good question that demonstrates that you’re thinking, and that you actually understand how complex evolutionary relationships can be.
Modern birds are monophyletic, meaning they all descended from a single ancestor that was also a bird. Since an important part of the definition of bird is that it can fly (or is secondarily flightless) that means that all birds are descended from a common flying ancestor.
In contrast (and to prove that your question isn’t stupid) there is still some tiny doubt as to whether the most recent common ancestor of all bats could fly. 15-20 years ago it was widely accepted that bats were polyphyletic and that the fruit bats had evolved flight independently from the ‘true’ bats. The last common bat ancestor was presumed to be something like a colugo that couldn’t fly at all. This hypothesis has becomevery much a minority view recently, but it still isn’t fully resolved.
And, just as importantly, there isn’t anything else descended from that common ancestor that isn’t considered a bird.
Perhaps slightly misleading, since the colugo (and presumably also the bat-ancestor) can do something which is a fair approximation to flying.
Nah, that’s completely irrelevant. For instance all living amphibians are monophyletic and the common ancestor of all amphibians developed from a tadpole. Nonetheless there are thousand of organisms that are descended from that common ancestor that aren’t considered amphibians.
Similarly if one line birds had since evolved into whales it woudln’t change the fact that birds are monophyletic and that the last ocmmon ancestor could fly.
Later descent changes neither the fact that he group is monphyletics, nor the traits of the common ancestor. All it means is that taxon isn’t strictly cladistic.
Gliding of the colugo type is in no way an approximation to flying. Maybe if the creature could ride thermals or otherwise gain or sustain height you might have a point. What the colugo does is to flying precisely what a rock does is to swimming: a slow but inevitable descent versus three dimensional movement.