Preening is the grooming behavior in chickens. Chickens smooth their feathers by running them through their beaks so they perform better at insulating and waterproofing. Chickens have one oil gland near the base of the tail, called the preen gland. When grooming, they rub this gland with their beak to release oil, that they then apply to their feathers.
Chickens preen on their own, but they really like to do it as a group activity.