I have two one 6 week old black australorp chicks. Last Sunday afternoon I noticed that one seemed to be limping, or favoring one side. I could find nothing wrong with her. She gradually got worse, and on Thursday I took her over to be checked out by the people I originally got her from. They've raised chickens for years. He gave her a thorough exam and could find nothing wrong; said she looked quite healthy and in great shape otherwise and thought it was probably a muscle injury. Today she is much worse - much more unsteady on her feet, falling over, and jerking her head around at times. She's still eating and drinking, although a little while ago she was breathing heavily. I have her separated from the other chick now, and she is contentedly laying on a towel. The farm where I got them from assured me that they do not have any Marek's in their flock. They vaccinate their chicks. I have two older birds (4 months) that I got from a hatchery (via a feed store). The older two and younger two have had only minimal interaction (never in the same coop). Any ideas what could be wrong with her?