We recognize different kinds of pecking. Gentle pecking we consider as normal behaviour and severe pecking as abnormal behaviour.
Gentle pecking: careful pecks, not resulting in feathers being pulled out and usually without interaction from the recipient bird. This is a social and explorative behaviour Severe/injurious pecking: forceful pecks, sometimes with feathers being pulled out and with the recipient bird moving away. This is clearly an aggressive behaviour.
There are stressful circumstances, which may result in aggression. If some of the birds start pecking aggressively (not all pecking is aggressive, much is occupational and non-damaging) it is usual to hear squawks of pain from the pecked birds. This needs early identification, as it is abnormal. It is an indication that there is a serious stress affecting the flock, and prompt remedial action is essential.
Loss of feather cover leads to increased heat loss and consequently to higher feed consumption.