I have a mixed flock of Barred Rock, black sex-link, Rhode Island Red and Cochins. My Cochins are the oldest in the flock and they are always in good feather. The others have good feathers for about a year and then start to look ragged. I feed a premium laying feed with flax seed and they are free range. I don't think it's parasites because my Cochins are never affected. My Cochins come from a small breeder and all the others come from a commercial breeder through my local feed store. Is it possible that they are just poor stock and more prone to feather loss?