Now as you surely will appreciate, we do have of differentials to consider for this situation. Now based on the distribution of loss and your further information, our main concerns are parasites (mites, lice, fleas), nutritional deficiency (especially protein),and possible boredom/behavioral causes. We can also see feather plucking in this area related to broodiness (though usually not multiple birds affected) and bullying (though that would depend on their position in those boxes since its usually the vent that is targeted) but these are less likely here.
Now you noted seeing no parasites, so we will assume those are less likely despite it being a consideration with this feather loss distribution. If you have any doubt, you may want to consider giving the coop a good clean out and treating with diatomaceous earth. That way we are lessening that as a potential factor if you are not 100% able to rule these out.
That aside, our key step in this situation would be to review their diet. You do sound to have them on a good ration but if we have a lot of extras that they are preferentially eating over the pellets that could be our root cause here. In any case, since laying birds need at least 15-17% protein in their diets and this is such a common component to see in deficiency and with these signs; I would suggest either trying them with a higher protein feed or even giving them a bit of dry cat food in their daily ration. As well or alternatively, you can consider supplementing with a supplement (ie.Lifeguard) that has trace minerals and some extra protein.
Next, we have to consider that there can be behavioral motivations for feather picking and plucking. The most common ones are related to boredom or a lack of ability to carry out adequate amounts of foraging behaviors. In the wild, chickens (well, jungle fowl) would have spent >50% of their day foraging for food. When we offer that food so readily, this leaves a lot of excess time for these ladies to develop vices. And often feather plucking is a vice that represents a lack of adequate foraging time. Therefore, to tackle this, it can help to feed finer foods as opposed to pellets. As well, that cat food I suggested before could be sprinkled in their litter or on the grass to let them have something to forage for and distract them from this.
Finally, while this doesn't quite fit with bullying, I would note that you can consider trying an anti-peck treatment (ie Pick-No-More lotion, Hot Pick Spray,etc) on the affected birds. This will help prevent pecking but if you use one with a color additive, you can see if there is a bully or whether the ladies are pecking themselves as we suspect. If you use this, do so after salt water bathing any skin that looks like it may break/ulcerate.
Overall, this situation does raise some concerns for the ladies. You sound to have ruled out mites/lice, which is good. From here, we need to appreciate the potential triggers for their signs and use the above to allay those signs. While doing so, as a short term option, I would also note that you can decrease light intensity and duration when they are indoors to reduce activity and thus the behavior. If we can approach this in a step-by-step manner, we can help halt this behavior and get the ladies back to proper feathering.
I hope this information is helpful.
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