Welcome to BYC. Google “mites and lice in poultry—TheChickenChick” for a good article to see what to look for for mites or lice, and treatments to use if found.
Chickens will peck and eat others’ feathers if they are bored from not getting outside, are overcrowded, or if they are not getting enough protein in their feed. What are you feeding? Try some Flockraiser or chick starter for 20% protein for a bit. If your chickens lay eggs, you can supply some crushed oyster shell for calcium.
That should be enough protein at 16%, but you can give them some higher protein feed or gamebird starter 24% for treats. Do yours get outside to roam every day? Even with the best conditions, some chickens will stand around and let another one peck their feathers. It can become a bad habit that can spread though, so just take a look at your room. Adult chickens do well with 4 square feet per chicken as a rule. If they get outside during the daylight hours, they usually spread out and are happier.