parasite or over amorous Roo??

[mufflerchick]

So I have about 5 hens that are showing bald spots on the backs at the base of their tail feathers...(sorry I don't have a pic). Most are a small circle about the size of a 50 cent piece except 1 of my RIR has quite a large area (about the size of a business card) that is bald and kinda red.....They do have an area under their coop in the run that they can go and have a dirt bath but I don't have any diatomaceous earth out for them....so I guess my question is...given the time of year (we're in MN)would you think this is an over zealous roo or parasites and if parasites.....would I NEED more than just the loose dirt under my coop for a bath??

 

[sourland]

Sounds more typical of feather pecking/plucking than rooster damage.