I have found out that I have a big bully! we got two chickens last spring, they've always been together and have always done great! we got two for free last Saturday, put them in the coop/run with the older ones and they did great for 4 hours while we watched. came back in 1 hour they were fine. came back 1 hour later and my two new one were pecked and bloody hiding behind the dust bath box. so I got them out and let them sleep in the building for a few nights while free ranging during the day. my husband and I built a partition on Tuesday (so it has been 5 days with the partition in the run) the first day the bully hen was walking around the run like terrorizing the new ones. it was almost dark and I could not get her back in the coop/run to close them up for the night, it took me 20 minutes to coax her in. she still does not like the new ones in the back of the run. if they come up towards the partition to peck at the ground the shoves her beak in there like shes trying to peck them again and they scurry to the back of the run. yesterday one of my new ones got around me when I was putting water/food in their part and the bully hen started pecking at her!!! how long will it go on like this? should I separate my bully hen for a while?? she is laying but the new ones wont be laying for a while since they are only about 14 weeks. right now they do not have nesting boxes or a spot for laying because they don't need it and I am hoping they will all get along. our coop and run is too big to build another one, I hate to waste all this space. like 10 chickens could easily roam and nest so I def think 4 should get along !
Here's a helpful guide for introducing new hens to old ones. It might help you. http://www.the-chicken-chick.com/2011/06/integrating-new-flock-members-playpen.html
I have two that are mean to the rest of my hens. They say to separate the means ones for about a month and reintroduce them to the flock. The means will loss there peck order when they are back with the others. I know that hens that get a taste of blood sometimes become blood thirsty. I sure hope you get this under control. I have one hen who is in the lower peck order and they won't let her eat unless I'm out there in the run, they pecked the feathers off the back of her neck. I plan on removing the nasty hens when I find coop to put them in. It can be upsetting when our flock gets disturbed.
Chickens enjoy a social convention called "The Pecking Order. This is a normal reaction when an established flock (like your first two birds) is invaded by outsiders. When you said "...they've always been together.... '' you hit the problem dead on the head. Chickens like to live in flocks of about 100 birds or larger. In this size flock there is little or no so called "bullying".
