It depends on the bird. He has to mature. And some cockerels never stop harassing the pullets. There are good and bad cockerels out there, give him time to mature, he could be a good one. But you really do need to separate him from the pulletsHow long should I let him keep on pulling on there neck feathers until he is just being mean. I will do this, but I would like him to try and get over it himself. Will he do it himself?