Do you have any idea why they are pecking at her? Is she ill in any way? When we got our initial 2 hens & rooster we noticed one had an eye infection. The other two seemed to peck at her and we figured it was because of the infection - they were trying to get rid of the possible threat to the health of the flock. by the time we started treating her eye infection, the other 2 had caught it slightly too so we had to treat all 3. now they are fine.
If you can put her in a smaller coop to isolate her - would it be beneficial to put another one with her? Would this help her be accepted back into the flock if she has a "buddy", even if she is lowest in the pecking order? Maybe others will know the answer. I've noticed that hens are quite social and like to be with other hens. We recently had an ill rooster (the alpha) and I isolated him with another hen. When we tried to separate her from him and put her back with the others, she couldn't be coaxed. We had to physically lift her away, and even then she didn't seem to happy. What do others think?
