1. How long has this been going on?
The bullying ramped up in early spring, and I thought it would subside once one of the males had established dominance. However, even now that the ranking has been clear for months, the winner seems to continue picking on the loser.
2. I have had good luck, at times, locking the bully up for a while (24 hours or so - you may have to do it for 2 days if the bullying is well established) and letting everyone else have a break and continue flock dynamics without the bully.
We put him into a separate area last weekend. He was frantic, but the rest of the flock calmed right down. However, he has turned into Houdini and twice broken out to rejoin them during the night. That's never happened before, and I'm still trying to figure out how he's doing it so I can close the loophole!
3. I have often helped a situation by making changes to the environment.
About a month ago, we doubled the size of their run, added more roosts and a couple hiding spots. It does help somewhat, especially for the low-ranking hen to stay out of the way. The low-ranking cock doesn't seem smart enough to avoid trouble. Adding more food dishes was a good suggestion that I hadn't thought of before. We did it, and I think it's been helpful.
4. Sometimes more free range time can "fix" things for us.
We try to let them out to range as often as we can. However, we live in an area with lots of predators, so someone always has to babysit them, which doesn't work well with our schedules. One day last spring my daughter was with them, and she went inside to use the bathroom. During the 5-6 minutes she was gone, we lost a bird. And it's hard to convince them to go inside before evening, even with treats. We've had other flocks that were much more cooperative and herd-able, but this little group is completely resistant and insanely frustrating. Once they're out, we're usually stuck outside with them for the rest of the day.
5. Something you probably don't want to do that may also help is to add more birds rather than take birds away, but that can be a double edged sword anyway...
This would probably help more than anything, but we're not going to continue with the guineas right now, and that would just prolong it. We are planning to transition to chickens. I've never had a mixed flock before, so I don't know if adding chickens would help in the same way adding guineas would. Thoughts?