Flock picking on the odd hen

[Beccichka]

Stats for background:
2 years ago we started our flock:
1 bantam Ameraucana rooster
1 bantam Ameraucana
5 Buff Orpingtons
4 Barred Rocks
5 Rhode Island Reds
Once they started producing eggs, we downsized by giving away 4 Reds and 1 Buff.
Last spring we hatched 1 bantam Ameraucana (and 2 roosters we did not keep once they came of age) and 2 bantam Ameraucana/Red mix.
In August we hatched a bantam Ameraucana/Barred Rock mix rooster. We got rid of him about a month ago as he was coming of age and too agressive with the hens.
This past summer we were given 2 Easter Eggers, 1 Ameraucana, bantam Cochins. The Ameraucana was always shy and kept to herself. The two Cochins rarely left the pen. We gave away the two Cochins. The rooster would chase the Easter Eggers away from the food sometimes or the treats I was giving, but they usually did ok.
All was well in our flock until February. In February, we had a big snow storm and they did not go out of their pen for 2-3 weeks. During this time, we noticed our loner Ameraucana was hanging back in the coop and eventually losing strength and laying near the food. We realized she was getting picked on. We moved her to her own mini coop and she recovered fine and recently started laying again. About 6 weeks ago, we lost our first hens to our livestock guardian dog. It was the first time they went out after the snow and there was still quite a bit on the ground. They did not make it back to the coop at dark and he got 1 Buff and 1 Easter Egger. In another week or two, he got off of his lead and got another Buff. A few days ago, our third Buff was snatched by something (not the dog) out of the forest when they were free ranging. Yesterday we bought 4 new chicks and decided to find a new home for the Ameraucana so we could use the small coop for the chicks. The last couple of days we noticed our lone Easter Egger who was previously very adventurous when she had her Easter Egger buddy and even after was hanging back in the coop. After what happened with the Ameraucana, we figured the same was now happening to her. And sure enough. Today, when she finally came outside, she was missing feathers and the rooster started chasing her and a couple hens started pecking at her. We have separated her into the small coop, but can't keep her there forever. We will probably try to find her a new home.
My question is. Does anyone know why this is happening and how to stop it? Must every chicken have a same breed buddy to not have this happen? Our Red has always been the only one and she seems to do some of the picking on others. Is it not possible to have a variety of breeds together or is there something aggressive about our rooster (who otherwise has never been a problem). I hope this doesn't happen to our lone Buff now that the others are gone.

 

[HollowOfWisps]

It is happening because the pecking order has been upset and your flock is in distress due to the dog attacks. They fell threatened, unsafe and positions of hierarchy that were previously filled are now vacant and there for the taking.

 

[Beccichka]

It is happening because the pecking order has been upset and your flock is in distress due to the dog attacks. They fell threatened, unsafe and positions of hierarchy that were previously filled are now vacant and there for the taking.

I just found the article here about isolating a hen that is pecked for a few weeks and then reintroducing her and seeing if she has regained confidence. We will try this. When we tried it with our loner Ameraucana it didn't work. But she always had a more reserved personality.

 

[Altairsky]

Your coop might be too small for all those chickens. Overcrowding is the most common cause of bullying.
I also would evaluate if the rooster is really a good rooster or not. Roosters should mate the hens, tidbit for them, not chase them away from food.

 

[Ridgerunner]

Must every chicken have a same breed buddy to not have this happen?

No. Mine do form cliques. Usually it is others they were raised with. I think their individual personality has something to do with it. Sometimes this is the same breed or color, but often not.

I've hear the phrase "birds of a feather flock together". Some people interpret that to mean that that a blue eyed yellow chicken with a rose comb only hangs with other blue eyed yellow chickens with a rose comb. My flock does not do that. What I think that phrase means is that chickens flock with chickens, ducks flock with ducks, bluebirds flock with bluebirds, etc. You can sometimes see a mix but to me that just proves that every rule has exceptions.

I also would evaluate if the rooster is really a good rooster or not. Roosters should mate the hens, tidbit for them, not chase them away from food.

I agree. A good rooster should take care of all of the members of his flock. He should only mate with pullets and hens with red combs so he is mating the ones laying eggs. Immature cockerels are not yet roosters so I make exceptions for them. Not all roosters are good.

Your coop might be too small for all those chickens. Overcrowding is the most common cause of bullying.

Again, I agree. Some of this behavior happened when they were trapped in the coop in the winter. The OP free ranges, I don't know how much of this behavior occurs when free ranging.

Does anyone know why this is happening and how to stop it?

We can only speculate. Crowding or a change in pecking order could be a root cause. Sometimes when a dominant chicken (male or female) loses its dominant position some of the others will expel or kil it to remove a potential rival. It could be temporary, it could be permanent. Another possibility is that a flock (including a rooster) will expel or kill a chicken they perceive as a risk. A sick or injured chicken could attract a predator or could pass along a disease so the flock rejects it.

How to stop it? If they are crowded in a coop or run give them more room. Letting them free range in the afternoon does not give them any more room in the morning.

Inspect the picked-on chicken for injury or disease.

If you notice one bird as the aggressor and causing the problem you can try isolating that bird for a week or so to knock it out of the pecking order. Sometimes that works, sometime snot.

Solve for the peace of the flock. Get rid of the bird being picked on or if you can isolate an instigator, that bird. There is a reason that bird is being picked on. I've solved that problem by eating that bird.

Good luck! This stuff is not always easy.