I have (had) a small flock of bantam cochins. They have been slowly dying - basically, they get a head tic, lose muscle control, and eventually die. No upper respiratory issues, no GI issues...just a worsening head tic and weakness. No rigidity like in Mareks, not affecting any other chickens in the other pens. Is this a vitamin issue? They are on a breeder crumble. I am down to one hen, and the kids adore her - and her roo misses her. Thoughts???