Feather pecking is a potentially serious issue in chickens. At best it can look rather scruffy, but at worst it can result in chicken death. Take a look at Peter Pecker in the photo. Notice his cream coloured feathers (his saddle) have v-shaped tips. This is classic feather pecking behaviour. The feather pattern of plucking is known as barbering.
The hens in his flock are being over attentive and because he is a kindly cockerel, he is letting them do it. He sees it as a mutual preening exercise which is a normal type of behaviour in birds. They often preen each other as it cements their bonds, however this can continue to a destructive level. Eventually the barbering gets closer and closer to the skin and if it does not stop then it becomes a feather pecking habit. Chickens will then start to attack the pin or blood feathers as they are starting to poke through the skin. This will draw blood and blood is bad. Chickens then begin to actively hunt feathers especially those of chickens that are lower down the pecking order. They do not know when to stop when they start to draw blood and it can turn fatal.
The reasons behind feather pecking are complex and not well understood. Some people believe that the hens realise they have a deficiency in protein and eat feathers to make up the deficit. Others think it is a boredom or a stress response. In parrots that is surely the reason but is it still the case for chickens? There could be a similarity for sure. Allo preening (bird mutual preening) is a bonding and very natural behaviour so we may be just guessing. It is all too easy to attach human emotions and reasons behind animal behaviour. At the end of the day we can only make assumptions but not know for sure.
One thing is for sure, when the birds are in a moulting period and every bird is in the same state of undress then they actually (mostly) leave each other alone and stop the feather pecking to allow the feathers to re-grow. Maybe it is more to do with the heightened stress of hormone surges during the peak laying and fertility periods of spring and summer.