I raise zebras and societies and have experienced this behavior with mine.Some birds develop this bad habit and it becomes a vice.It really has nothing to do with nesting.I usually separate them with a breeder cage that has a wire partition so they still see each other and cant pluck each other till the feathers grow back, then reunite and if they continue you may have to accept the fact that it may never stop.It can help to put special "preening toys" in the cage made of sisal and feathers to get their attention shifted.It is usually boredom related but sometimes is an overzealous male picking on an uncooperative female, she will be bald on the neck and back if that is the case.Too much overcrowding like in pet stores always results in tailless bald butts on birds.