My hens (17) will be a year old next month. I have one hen (my favorite) that has begun to act listless, stands off from the rest of the flock, shakes her head back and forth in rapid motions and also keeps it tucked down in. Her feathers appear to be fluffed up constantly. I know this is like performing brain surgery over the phone or something, but would anyone have any idea as to what may be wrong with her? She used to be so active and was one of the first to come to me when I went into their coop. I also do not think she is getting up into the nesting boxes or on a perch, either. Any help would be most appreciated.