I have lost a lot of birds both to hawks and dogs. I have leghorns which really seem to bring in the hawks for they can see them the best. Mostly, I have coopers hawk come and get the leghorns they have tried to get one of my barrock but wasn't as successful. So probably the bigger the chicken is the more it can fight to get away and also I have had red-tail hawk which could get my bigger chickens. But I would probably suggest have good cover for your girls to hide under that hawks can't get under very easily either for the hawks I believe have to land on the hens back to be able to kill them. I tried some roos but they just upset my hens to much so I got rid of them. But I have read that some have really good success with roos.