It's definitely in the bone structure; the back makes a strong "c" curve, which makes the tail follow the direction of the curve in the back. I guess that could be affected by underdeveloped musculature to some extent, but would think the muscles would be underdeveloped on both sides. I have no idea how long it has been known in silkies, but I understand it has become more prevalent since they've been more carefully bred for short backs. I've got a beautiful cockerel that would be a show bird if not for the curved back. I sell them as pet quality, strongly urging they not be used for breeding. I may bring him to Shawnee so interested people can feel the curve in his back. I think I've only had two wry tailed birds this year out of a couple of hundred.