There are a few things that will cause poultry to eat feathers.
There feed is lacking in Protein/s, (Methionine, Arginine, Animal based Protein)
Mites or Lice,
Low mineral level in the diet,
Boredom
Dust your birds and then try supplementing with some high protein game bird feed. One that is around 20% to 22% protein and that contains Animal Proteins in it.
Here is a quote from Dr Marinus van Krimpen of Wageningen University.
"Severe feather pecking has been reported in birds that were fed a too low mineral level in the diet, a too low protein level or a too low amino acid level (methionine, arginine).
There appears to be somewhat more more feather pecking with diets containing only vegetable proteins compared to those fed some protein of animal origin.
Feather pecking is also more associated with diets that were restricted, coarsely ground or fed as pellets."
Dr Marinus van Krimpen of Wageningen University
Chris