Wry tail in silkies is caused by a curved spine, not musculature, which, of course, may be different from other breeds of chickens. You can easily feel the strong curve in the spine. The prevalent belief is that it is genetic, however, I've read other theories about wry tail; that it may be caused by outside factors such as too much incandescent lighting or too high protein at too young an age. A medical doctor who raises excellent quality silkies feels strongly that it is caused by too high protein, causing growth to be too fast. He swears that he sells his wry tailed birds to a lady who breeds with them and never has wry tailed young. I'm doubtful of that, though. He also insists that they sometimes grow out of it, which I didn't believe until I had one young cockerel whose back was starting to curve, but it DID straighten out. He was only a few months old. It usually becomes evident in my silkies around 3 to 5 months.
I read an extensive paper online from a group using chickens in a study of scoliosis in humans. It concluded that one causative factor might be sleep deprivation caused by too much exposure to incandescent lighting. That makes sense to me, as many, many chicken breeders use incandescent bulbs as heat sources, 24/7. I have occasional wry tailed silkies and don't use them in my breeding programs at all. It is something you almost cannot get away from in silkies, possibly because of the breeding for short backs. I have fewer since I cut back on the protein in my younger birds. I also quit using incandescents for heat several years ago. I use the black/purple heat bulbs for reptiles. They are more expensive, but they do seem to last longer as well. I think there is nothing you can do about wry tail once it has formed.
