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Your bird is probably plucking and eating feathers because she is lacking protein. Feathers take up 85% protein and when chickens molt, they need extra. So, that is why she is eating the feathers. You need to stop her from doing this as it can cause crop problems.
Give them foods such as mealworms, quinoa, peas, meat scraps and oatmeal. These will all help them get the extra protein they need. Mealworms are possibly the best you can feed them.
Check the one bird's crop for the next few nights also to make sure everything is passing correctly. It should feel hard at night and empty in the morning.
Good luck and welcome to the flock!