We have a 3 yr old B. Rock who started stumbling 3 days ago with the occasional falling over. We checked her over and can not see anything else wrong with her. She is in with 14 other chickens and no sign with the others. The weather here is mild in the 70 so I don't believe it is weather related. She is eating and drinking fine and when let out to free range she does so without any problems other than the stumbling. Any thoughts ??