New owner this year. We have 5 birds around 5 months of age and 6 birds that are about 2 months younger. One of those new birds was a runt (Cochin) and looked like a new chick well after the others tripled in size and feathered out. We kept that Cochin inside longer since it had no feathers and the others went out to our coop in a dog kennel for one month to get acquainted with the big birds. They all get along fine after the initial pecking order was established by my cockerels (have two right now, debating getting rid of one). Well over the 4 weeks the Cochin finally started growing rapidly. It (still not sure if he/she) is now the about same size as the other younger birds but still the runt of the group. We put this bird in the same kennel for 2 weeks alone in the coop to get to know the others. Tried taking it out and it was instantly attacked by the bigger birds and even some of the others the same age. Not just "pecked" but pinned down and plucked by several others! Poor Cochin lost a patch of feathers before we could get the others off. Put Cochin back in kennel for another week. Took it out and things were good for a couple days. Now it is back to the same issue - the big birds and still one or two of the others constantly harass this one. The Cochin keeps trying to hide his head under somewhere or duck under a bush and will get attacked from all sides. It just keeps squawking and running away like crazy. This is pullet and cockerels going after the Cochin. I've watched the others peck at each other and it seems "normal" to establish things. Put frozen fruit out and the cockerel goes first and the others come in later with smaller birds eating last type of thing. If others get too close the boys put a quick peck on them and all is fine. This is different. They corner the Cochin and once close enough fly up to grab it with their feet to pin down and then they start pecking it's back and neck. The Cochin is about as docile as can be and will come running towards us when we walk in the run and hide behind us or sit on our feet if we are standing still long enough. Anything we can do for the poor bird? I dont want it injured and it has been near a month now in the kennel. Several birds leave it alone all day. The others... it gets ugly fast. Right now when it happens we get after the other birds (tap at their tails and right behind their neck) and keep them away while keeping watch over the Cochin.
If the other chickens are aggressive breeds, you have a nearly hopeless case. And the Cochin being a docile breed, it's doubtful you will be able to instill enough self confidence in this chicken to enable it to stand up to the aggression. But you can try. This article of mine will tell you how to go about it. https://www.backyardchickens.com/articles/chicken-bully-chicken-victim-a-two-sided-issue.73923/ Other things you can do to level the field is to install high perches in the run, perhaps a shelf or a chair for the Cochin to rest on with food and water up there so there's little need to compete at the feeders. If you don't have ten square feet of run space per chicken or more, lack of space increases aggression.
Introducing a single bird is tough, EVERYTHING knows this does not belong. Try picking one of the birds that does not attack her. Put them together, wait till they are friends, do not rush this. Next pull out two of the meanest birds or three, put them where you have the victim and other bird, while you put the victim and friend out with the flock. Pray this works. Wait until you are sure the victim is holding her own in the main flock. The ^^ idea of roosts and hide outs, platforms and extra feed stations is a good one. Then add one of the aggressive birds, one at a time. You want the flock to be tough on her, so she is so busy defending herself she does not attack the victim. Gradually add the other birds. If this does not work, you need to pull the victim from this flock. It is funny but that violence is also hard on the birds not involved. It keeps the flock upset, always solve for the peace of the flock. Mrs K