Feather-pecking is a nasty vice which is difficult to deter.
Pheasant poults are very guilty of it when reared in large numbers...so much so that they are often fitted with either plastic 'bits', which are 'C' shaped and fit into each nostril, passing between the mandibles, and serve to keep the beak from closing properly so that feathers cannot be gripped; or 'specs', which are two discs which sit above the beak and don't allow the bird to see anything directly in front.
These devices are available for poultry too.
Birds which feather-peck are greatly attracted to new feathers in their early stage of growth as these are full of blood. If the supply of these runs out, the birds continue to peck at the damaged skin around the tail and vent and can make a real mess of the pecked individuals. Often, culling the feather-peckers is the only way to wipe the habit out in a flock.
Eating feathers from the ground, which have been lost or moulted, is a different thing and, I believe, is is related to the crop and digestion.