Primitive birds (basalmost paraves) flapped their wings and flew using the same set of muscles as their pterosaur ancestors.
|Furthermore, the supracoracoideus muscle,|
and hence an ossified sternum, is not necessary to effect the
recovery stroke of the wing. Thus the main evidence for
Archaeopteryx having been a terrestrial, cursorial predator is
invalidated. There is nothing in the structure of the pectoral girdle of Archaeopteryx that would preclude its having been a powered flier.
|Concerning the lack of asymmetric feathers in flying basalmost paraves, the following seems relevant:|
In modern birds (Neornithes), the wing is composed of a layer of long, asymmetrical flight feathers overlain by short covert feathers [1-3]. It has generally been assumed that wing feathers in the Jurassic bird Archaeopteryx [4-9] and Cretaceous feathered dinosaurs [10, 11] had the same arrangement. Here, we redescribe the wings of the archaic bird Archaeopteryx lithographica [3-5] and the dinosaur Anchiornis huxleyi [12, 13] and show that their wings differ from those of Neornithes in being composed of multiple layers of feathers. In Archaeopteryx, primaries are overlapped by long dorsal and ventral coverts. Anchiornis has a similar configuration but is more primitive in having short, slender, symmetrical remiges. Archaeopteryx and Anchiornis therefore appear to represent early experiments in the evolution of the wing. This primitive configuration has important functional implications: although the slender feather shafts of Archaeopteryx  and Anchiornis  make individual feathers weak, layering of the wing feathers may have produced a strong airfoil. Furthermore, the layered arrangement may have prevented the feathers from forming a slotted tip or separating to reduce drag on the upstroke. The wings of early birds therefore may have lacked the range of functions seen in Neornithes, limiting their flight ability.
Longrich NR, Vinther J, Meng Q, Li Q, Russell AP.
Altogether we have a picture of a flying, feathered, 4 winged, arboreal, primitive bird with a long bony tail, that flew like a pterosaur. With elliptical wings and symmetric feathers
At times people have argued that a flying pterosaur would not devolve into a gliding primitive bird. But this is misguided, because the evidence indicates that flying pterosaurs evolved into flying primitive birds, not into gliding primitive birds.
Here are the aspects related to flight capability:
- muscles used
- keeled or not keeled sternum
- flight feathers (asymmetric or not)
- feathered hindlimbs
See here for more details:
A stiffer feathered postpatagium in Anchiornis may have compensated for its aerodynamically inferior arm feathers to some degree