Featherless spots below my chickens' vents. Mites? Cannibals? Help!

[mamachook]

I have four black Australorps. A few weeks ago two of my girls suddenly appeared with bald spots on their bottoms below the vent. These patches were not bloody or anything, just sort of bumpy like, well, chicken skin. A third now has a bald spot too, but hers is sort of scabby/bloody with feather stubs.
I haven't seen any signs of aggression or cannibalism, but I guess there's still the possibilty that they are picking each other's feathers off. They are on a combo of scratch and layer feed with oyster shell mixed in. They have a roomy run and are let out into the yard to wander for an hour or so every couple of days. Between the four of them, they are laying about 2-3 eggs per day.
I haven't seen any feathers lying around the coop or anything and haven't noticed any mites on them. All of their patches are in the exact same place.
Are they pecking each other, even though I haven't seen this? Or is this like chicken mange or something? Mites? What should I do? Please help me out, I don't know what to do.
Thank you!
Oh, and I live in Hawaii, so we don't have possums or foxes or anything wierd like that.

 

[kelar]

You could have a feather plucker in the group, but I've found that these culprits usually go for the head feathers. This is most likely mites or lice or both, which cause the bald spots & redness around the vent area. Sometimes you don't see them because they spend a lot of their time OFF the chickens, but if you check at night with a flashlight, you will usually catch them in motion. Karen

 

[mamachook]

Hmm...I checked at night for any critters on my girls and didn't see anything. I'm reposting my original question with this fact noted and I'll see if anyone comes up with anything. Thanks for helping out!