How old are these birds and are they both hens? We need more info to help. My initial thought is you need to step in and establish order (just like a roo). I had a nasty hen fight when my broody was returned to the flock after raising chicks and it was a bloody mess. Hens can be very aggressive when it comes to establishing order. The roo usually fixes these things, however not always. If they continue to fight I would encourage you to separate the injured girl to give her time to heal. Altho, beware she may very well go thru this again when you return her to the flock.
I would get rid of it. my 2 Amerucanas (well, EE) were my worst. The roo was almost evil to the other chickens (we re-homed him) and my girl is just a touch anti-social (where everyone else is lovey and snuggly).
If they are both only 3 weeks old, they are probably learning how a flock works. I think removing the bully was a good idea. I would add her back in later and see if she can behave herself. Keep an eye to make sure they behave. In all honesty, I have never had 3 week old chicks fight like you explained, so I can't say for sure how this will turn out for you. I would not get rid of her. She is only 3 wks old and really a baby. I don't think this is any sign of future behavior. Let us know how it goes when you put her back. Sometimes they just need a time out to realize it's much more fun to be with the group. Good luck.
Thanks, I just moved her back and there doing okay so far this is only my second time raising chicks. It seems like I have more trouble this time than last. I moved my little rooster and m dominiker bonnie into another booder just for a little bit. Bonnie as been challgeing the americana for awhile now, so a guesse she just finally got mad. I was looking around bonnie to see if there was any wounds and it looks like her neck area is sunk in. Is that her craw.
Its a little less crazy now.