I am the first to admit that there are problems with this scenario of secondarily flightless protobirds. (walking dinosaur costume)The worst is that in some ways some protobirds are less birdlike than archaeopterygians and dromaeosaurs.(realistic dinosaur costume) In particular, oviraptors, ostrich-mimics, troodonts, and avimimids seem to have normal and vertical pubes like most theropods. Perhaps this is a reversal, for unlike birds, proper protobirds still had good tails and pubes that were behind the zuts instead of on either side of them. If thev reenlarged their tails, they no longer needed the belly under the hips for balance, so it might have been swung forward again along with the supporting pubes.
There may have been fewer cases of secondary flight loss in the protobird clade than I suspect. Instead, the nonflying protobirds may have experienced massive parallelism with the main body of flying protobirds. Another possibility is that flying protobirds continued as their own clade, competing with the true birds (including, possibly, “Protoavis”) and spilling off flightless forms during the Cretaceous.
A vital point about bird origin is that the increasrngly avian design of Archceopterry and cretaqeous protobirds makes them, in the whole, the best potential ancestors of birds, much better than protobirds. (Animatronic dinosaur)