I'm posting this here too hoping someone has an idea. My young RIR is missing her neck feathers. At first I thought they were just slow growing in after the down fell out but it's been way too long for that. She's healthy except for this. I've never seen anyone (two other chickens, one close to her in age and one adult hen) pick on her and I watch them a LOT. She and the buff get into "chest bumping" spats but no one messes with her head and neck. Not even when I've checked them at night. She dust bathes regularly and I keep DE down I the bath area, and I haven't had any mite/lice issues in the past. What could be causing this?!