Best ways to apply Blue kote for feather eating?
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Do you have photos of the bare bottoms?
Are they drawing blood when they pick/pluck one another?
Are they going outside at all?
A higher protein feed like feather fixer will help if they are eating them and need protein.
Boredom can be part of the problem as well. The flock block can help, also you can hang a cabbage, some kale, apples, etc. in the coop or run for them to pick at.
I would recommend that the treats don't exceed 10-15% of diet since you are trying to increase protein.
As far as applying bluekote, you can either spray it on the red/exposed areas or use something like a qtip/swab and dip it in the bluekote to cover the exposed areas. Bluekote probably won't keep them from plucking and eating feathers, but if there are raw areas it's antiseptic properties will help cover and heal.
Some people use a product call "pick no more" to apply to feathers, it's supposed to taste bad and discourage the bad behavior.