Welcome to Backyard Chickens, sorry I don't have an answer to their feather plucking. They may just be bored. You could try hanging a head of cabbage or lettuce just out of reach so they have to jump up and work at getting some.
Hi and welcome back to BYC We're so happy you've decided to join us
Maybe ease up on the corn. Corn is like candy and full of wonderful stuff but only about 8% protein. Maybe offer some fish as a treat instead. Cheep canned salmon or mackerel are wonderful in moderation and a huge boost in protein.