I will jump in here and say I just had an experience with very new ducklings. One was about 5 days old when the next hatched, got along fine, but was quite obvious the first was a male. Then 5 days later, another hatched, tried to introduce her and the boy darn near killed her. Took her out, put her back, over and over, but the same thing happened. Removed the male into a separate box, he is none too happy, but the other two are getting along just fine, so he is a villain. Threatened him I was going to put him out with the big girls if he didn't behave, but of course he didn't listen to me. So he will live a solitary life for a few weeks. You never know, there may be just one who gets things stirred up, and the rest go along with the bullying. They can easily maim or hurt a newborn, so constant vigilance is necessary for the first few hours. If everyone is getting along at that point, you are probably home free. Good luck! This was my first bullying, and I was appalled at his behavior, but maybe he will come around if he gets lonely enough.