The challenge I had with the anatomy was to create a way of making a back set of wings flight worthy. Because shoulders and hips both have ball-joints, the base of the wings was fairly easy. After that I replaced the usual knee joint with what was, essentially, another elbow. Because Quadornis now had no legs, I had to devise a way of it getting around so I slightly elongated the claw on each of its wrists. The claws would allow the bird to crawl around and climb on other animals but, because it now had no legs to launch itself from ground level, it would be restricted to climbing to a high point and basically letting itself fall into space in order to become airborne, much like swallows and swifts.