I think all chickens are like that, & it's particularly difficult if you add one one at a time to the flock. Can you put the new one inside a portable dog pen imbedded inside the larger chicken run? Let 'em get to know each other with the protection of a fence between, so it's a bit more gradual?
Mine are just a bunch of hooligans. I dread the day I need to introduce my leghorns into the flock, but it will be soon.
