Bully Hen

If the aggressive one is the only mean one I would probably make dinner with her.

 

"Nasty birds taste best!" At least move her on with full disclosure. Mary

 

It sounds to me like you've got an excessively aggressive bird on your hands. Is it one of the originals or one of the newbies?
I believe it likely she's just got a nasty temperament, I've had a few like that, and they all moved on very quickly.
There are a couple things you can try though, if you're not quite ready to cull or sell the aggressor.
1. Make sure to have multiple food and water stations, birds who feel like resources are limited are driven to protect said resources. Keep food available during all waking hours at least.
2. After making sure the last is done, aggressor bird is separated in dog crate, and victim bird is no longer bleeding/bloody (chickens attack blood, will cannibalize one another) let victim out with the rest of the flock. Supervise and make sure no others go after her. If all is well you can remove aggressor bird crate and all from sight of the flock. Move crate to a safe location and keep her separate for a while, I believe a week is a commonly suggested tine frame. This works by forcing her to renegotiate her position with the whole flock, and distracts her from attacking victim bird. In theory. This still might not work, but is worth a try if the rest of the flock seems cohesive without her. If it doesn't work, and especially if her antics rile other birds into aggression, as I've experienced in the past, she needs to go.
Best wishes.

 

Since she's one of the newbies, that changes the dynamics a bit. More food and water stations are always best, but I'm beginning to doubt that it, or separation, will help with this bird. It's worth a try, I suppose.
Chicken pecking order dynamics can look pretty offensive to us, and sometimes certain birds do go over the top. If you separate her, when you reintegrate her be available to watch closely for a while. She will likely attack again, I would try to refrain from getting in the middle of it unless she draws blood. I bought a beautiful pair of crested cream legbars once, the tiny little hen spent the first 20 minutes out whooping every bird in my flock's butt's. She didn't draw blood, and she moved on from each individual as soon as she saw submission. It was disheartening to watch, but she remained head hen for the rest of the time that I owned that flock.
Your bird seems to have the same intention, and if she was housed with another flock before, it's possible that once she's sufficiently (in her eyes) proven her dominance she will chill. Another factor to think about is the rooster, after a week or so (approximate separation time of bully hen) he will likely be more inclined to protect new hens from abuse. A good rooster prevents much squabbling between his ladies. During first integration the preexisting flock was not his ladies, now they will be. He should be an asset.
If the second integration ends with bloodshed again, or if she persistently bullies old flock birds, I'd sell her (with full disclosure) or give her away, or eat her if that's something your family is up for.
Also, if it were me personally, I'd separate her for a week, out of sight from the flock, but then the day of reintegration I'd put her crate in the run and let her be around them but unable to touch for a while before letting her out.

 

You're welcome, I help when I can. I've spent massive amounts of time observing chicken behavior, taken animal behavior in college, and done obscene amounts of reading on here. I value peace amongst my flock above all else, I wouldn't keep the most gorgeous egg laying machine if she caused chaos or unhappiness among my birds. I've always culled for disposition and had peaceful happy birds because of it. While you may not want to give up on her, the rest of your birds depend on you to keep them from harm, even if the harm is one of them. I'm sure you know this. I've successfully rehomed a couple nasty hens, and it seems, while you should usually avoid adding a single bird at all costs, with the nasty ones it's often just what they need (which is what the separation is meant to simulate). When they alone are the outlier, they have to be a bit more busy defending themselves and have less time to plot evil
I hope your rooster takes well to being a flock leader, submissive roosters can sometimes show unappealing characteristics once they're out from under the rule of another. And the other end of the spectrum, sometimes they are the submissive cock because of underlying issues like low hormones-have noticed personally and read of small gonads on these overly submissive cocks upon processing for their meat after they've failed as flock guardians, in my case was below most of the hens in pecking order.
Hope yours steps up as flock guardian and doesn't get too Randy with all his new-found alone time with the hens. Perfect roosters have been few and far between in my experience, although any "man fighting" issues can be trained out, others like over mating, rough mating etc are hard to deal with and result in looking for a replacement around here.
Best wishes, update this weekend on how it goes if you want, I love resolution!

 

Hope it works, CJP: if not, you tried. They can be little velociraptors at times. Mary

 

Wishing you luck with the nasty hen...I'm dealing with one right now and it has gotten out of control in my eyes.
Our problem hen is a 2 year old Plymouth rock and she constantly chases..pecks and scares my other docile hens daily. What made my mind up about getting rid of her was when I watched out roo (lavender orpington ) mate with one of the other hens, she came charging over and repeatedly pecked at the hen's head while roo was still on her! I was very upset to see this...the docile hen had no way of avoiding the pecks to the head...and she keeps doing it. So a friend of mine who breeds and raises chickens said she will take her...I just hope she doesn't do the same to her hens!
So there's my story of boss hogg hen we have lol