chickens are picking on one hen

[borzoimom]

I have a small chicken flock, 5 hens and 1 rooster. They have been together their whole life, since last July. All the hens are laying, They spend their nights in a coop with attached run and during the day they free range on our property (5 acres). Three days ago the rooster (Favorolle) started attacking one of the Australorp hens, I mean really going after her (wings spread, head down, feathers puffed and grabbing her anywhere he can get to, it definitely is not mating) Then yesterday, the favarolle hen began pecking the same hen if she came too close to her. I checked the Australorp for anything amiss. The only thing I can see is she now has a bald spot on the back of her head, funny thing is it is not bloody, but red and textured like a comb, but not raised! But it is a good 1/4" behind her intact normal comb so unsure of what that is ( I first noticed the red spot about 2-3 weeks ago, Back to the rooster; the poor hen either stays in the coop or out in our yard alone during the day whick worries me that she won't have the protection of the flock. I don't want to lose this hen as she is my best layer. What is going on and what can I do? If it makes any difference, the flock consists of 2 Australorps, 2 Favorolles, and 2 Cochins.