strange pecking problems

I have 52 little baby quail, they keep pecking 4 or 5 of the slightly smaller ones in the butt. The smaller ones are at most only a day younger. Not just a little peck now and then, it's like they're bullying them. They have made several bleed pretty badly, so I took the smaller ones out and put them in another brooder and the pecking has stopped. None are over two weeks old. Someone told me that when they do this they are lacking in some important part of their food. What could it be? I'm feeding them the starter grow.