Check at the base of the feathers around the vent for clusters of nits... It'll look like tiny clumps of white foam. Those are eggs. You might also see little pieces of cayenne pepper run away into the surrounding feathers... Red mites are nasty beasties.
If you see any of that, grab some Sevin dust from your local garden/home improvement store and give your ladies a dust bath around the vent, under their wings, along their backs, and up their necks as best you can. Do this every seven days for three weeks.
Absent of external parasites and roosters, I'd say you've got a bully chicken that's either mounting your hens as a show of dominance or pulling out their feathers.
You can try covering up the area with iodine or Blue Kote to hide the tender, pink skin... Having angry tissue exposed like that is a target for the flock. You could also opt for a Hen Saddle to help the feathers regrow and give them a little protection from the elements.
Other than that, intense observation is needed. Grab a chair, a book, and prepare to sit outside and wait until you see the behavior. If they are picking and EATING the feathers, you could have a protein deficiency in the flock.
MrsB
