By nature chickens peck each other and some pecking is normal. If you are mixing a new group of chickens they are going to fight until they get the pecking order established. However, when they pick all the feathers off or kill each other, it could be brought on by several reasons, some are: they may be too crowded, they may be too hot, you may have too much light on them or light on too long in the night. Be sure you are feeding them formulated chicken feed (if they are not getting the right nutrients they will peck each other) or not enough roost space. We recommend drinking nipples for chickens because of the shiny metal tips that they like to peck at which keeps them busy and makes them drink a lot of clean water for good egg production.