Sadly, there's not much you can do to protect a chicken who seems to be at the bottom of the flock pecking order, and that is pretty much what "chicken bullying" is. It's how chicken world works.
I'm about as close an observer of chicken behavior as you can get, even admittedly being fanatical about it. And I've concluded that there are just some chickens, by nature of their individual personality, who are destined to be chased, pecked, run off the feeder, and generally treated "like dirt" by their flock mates.
I've tried to referee, mediate and intervene, and nothing I ever do has changed things in the least for the down-trodden victim I was attempting to help.
Chickens have an extremely fluid, yet rigid social system, which to us may seem brutal and unforgiving, but to the flock, it allows them to function as a close-knit social unit providing security and well being to each member as they fit into the whole. It's extremely delicate, and most attempts to change what you see as "bullying" will mostly just make things worse.
