One of the others is picking out her feathers. Probably the one next to her on one with unharmed feathers. They will sometimes do it on the roosts while relaxing or getting into their favorite spots. I have used BluKote or Nustock cream to prevent pecking of bare spots. Make sure they are getting a good balanced chicken feed at least 16%, or better 20%, since a lack of protein may make them want to eat feathers. Overcrowding, not enough nest boxes, or boredom from not going outside can be other possible causes.
You should look them over for any lice or mites moving about on their skin and laying eggs. But feather picking looks to be the problem with the black spots and broken off feathers. Look for one that doing have any missing feathers, since that may be the feather picker. It can become a habit that spreads to others. As I pointed out in post @, there can be several possible causes. What are they eating? Are they getting a balanced layer or all flock feed? Are they eating the feathers when you are around? Are they getting out to free range? Are they overcrowded? Are they bored or do they have ways to entertain themselves? In the article below, is what to look for if there are lice or mites on the skin under the vent and elsewhere.