Before the feather eating started, they were getting a treat maybe twice a week.  Definitely not anywhere near 15%.  Their food, at 21%, should have been sufficient,   But they were/are eating feathers.  The amount of protein was increased only after the problem was noticed. I do think that there might be some improvement in the amount of feathers being eaten, and they have not progressed to pulling feathers off from each other or themselves.  I'm wondering if the large feather loss (probably due to growth?) might just have caused a greater need for more protein.  Kind of a growth spurt thing?