bullied finch

Question
I have 4 female finches, and have had all 4 (siblings) for about 3 months.  I just noticed today that one has a bare patch on her chest and behind her head, and I think I have figured out which bird is the troublemaker.  She assumes the aggressive posture of stretching out her neck and chases the other birds, but especially the one with the bare patch, off the perches and nesting baskets.  What could have caused this aggressive behavior after all this time of peaceful co-habitation, and if I isolate the bully, is there any way of re-introducing her to the larger cage with the others in the future?

Answer
I'm not entirely sure what causes a bird to go mean all of a sudden but it's not hopeless. There is a small chance of you being able to get her back to her old self. It's not for sure but it has worked in the past. What you want to do is to remove the trouble maker from the cage and room. Make sure your bully isn't where she can see or hear the others. Keep a very close eye on her and after a couple weeks put her cage next to theirs and watch her reactions. Keep here there for another couple of weeks and if she's not being aggressive then you can put her back in (watching her closely though). If she is you have to start all over.