Just wondering about one of my bantam chickens.She has lost all her underbelly feathers and then some on top of her wing and thenin a circle around her neck like a necklace.The actual quills are still in her in some parts of her body but underneath she is bald and has what looks like one little sore.There are no roosters in the pen and she is not at the bottom end of the pecking order and has never been picked on by the other 5 hens.They have been locked in their pen for a week while it rained and they usually get to come out everyday for a roam around.She seems a bit sluggish and is off her food.I have checked her for mites and ticks.

All I found online was that it could be mites, stress, or the other birds picking at her. It could be a sign of aome other illness. I'd have the vet check her.

Turns out there are a lot more possibilities than I thought..
The site below details all about feather loss, the causes and treatments.

Feather Loss In Pet Birds