[andreadon]

Don't worry about it too much: our ex-batt Rita still has a mostly bare bottom: we call her pyjama girl because she looks like she's wearing pyjamas where all the others have lovely petticoats.
her bum is sometimes a bit red and warm too, and i think someone's already mentioned it might be from sitting in her own poo....  try to make sure she always perches when she's gone to bed (even if you have to lift her there)
In theory her feathers will growe back in time!
Rita was bare-bottomed all through winter (and we had to wash her several times because she's lay her eggs with no shells) and she was fine.