If there is one bird that is continually getting picked on in a flock, it is usually for a good reason. The flock are wanting to eliminate her for some reason, usually it's a health issue. You can't kill the whole flock for picking on the one bird, so the solution is obvious though not many want to do it. Everyone wants to fight for the underdog but in nature the underdog is a weaker, less desirable animal in the gene pool and the others will cut it from the herd or even kill it for that reason.
Here's the choices...you can isolate her and doctor her back to health but then you have to re-introduce her to the flock which, no matter how you do it, may again make her a target for aggression. You can Blukote her or otherwise cover the site and leave her in the general population but they may resume picking on her again once she is healed. You can try to change the flock matrix by penning this or that bird with her during her isolation but you have no assurances that bird won't peck her also. Or you can eliminate the problem and her pain and misery by culling the bird~ it takes all the guess work out of it all, provides mercy for the bird and also eliminates your work, worry and stress over one bird.
If you had several birds having a picking problem, it can be space or a breed issue. If you have just one bird that is singled out from the flock, it's a bird issue. The former is a little more difficult to assess and resolve than the latter. A lot of people will tell you to keep her, nurse her back to health, change this or that to try and fit her into the flock, will give you all kinds of success stories about how they changed the life of a bird just like that by doing such things.
The only thing I can tell you is that I've never had such a problem in any of my flocks because I cull yearly for laying, healthy appearance, and behavior....and that seems to remove misfit birds, those that would be aggressive enough to peck holes in another one, birds that would develop health issues and also any bird that is too weak to defend herself in a flock society. A lot of people think culling is cruel but it's rather the opposite...it prevents things like what has happened to your poor chicken. I've never had to walk out and see a bird with a bloody behind from cannibalism and aggression. Any old timer in poultry is usually a proponent of judicious culling of the flock to improve overall flock health, genetics and social interaction...that's how they become old timers in poultry.
