It could be peck marks, but it may be some sort of illness also. Thanks for posting a pic, BTW. I don't want to hazard a guess as to what exactly happened there.
So a lot of the time, if there is a sick or injured bird in the flock, the rest of the birds will gang up on him/her. It is totally normal chicken behavior. Personally, I would separate that one and let it heal up, or if it is something besides injury it's a good idea to separate the bird anyway. Yes, reintroduction into the flock will be difficult, but if that one is getting picked on continually it's probably a good idea.
Some of the roosters might need new homes, once you figure out their sex for sure.