Its been months but I have finally fully integrated a single chicken into my existing flock. It took a lot of effort, stress and time but I finally did it! The new girl went from getting a chunk out of her neck bitten, to sleeping, eating and foraging with the others. Advice I would give, 1) For two weeks or so put the chicken inside the coop in a smaller cage (allow the chickens to get used to her but not touch her) provide food and water, allow her out to forage, she'll avoid the flock. Then put her back in. 2) After two weeks at night put her in the coop, where the other chickens sleep, that way they wake up with her, as if shes always been there. 3) Have a perch in the run, to avoid fighting the new chicken will jump up there and be safe, the others might join her, but there is no fighting on the perch. Allow them out to forage, take the new chicken to get food and water in a quite corner of the garden. Each day! to ensure she eats enough. 4)At night put the new chicken in with the others, at the beginning when shes still not really a member of the flock she'll sleep in the run, give her some confidence and put her with the others. 5) Be patient each day you'll see the new chicken get more confident, use good treats to get the other chickens to forget about the new comer and just eat, once you get them to eat side by side you've truly broken ground.