Jay,
Unfortunately this is the time of year that all predators are looking for a free and easy meal, chicken is on everybodys menu! You are doing the right thing by being out there while they are freeranging and having your dog there as well. I would stick close to them while they are freeranging and lock them up when you cant. I am having the same problem with a hawk and a falcon so nobody is out unless I am right there with them. I have also had a coyote make a try and some bears pass through. At some point I will probably lose a chicken to a predator but I do my best to make sure that I am a diligent deterent. This fall has been the first time I have had any predator problems and an abundance has shown up this year. I have found that being close to the cheeps has been a great deterent to keeping the hawk and the falcon away. Like I said, chicken is on everyones menu so stay close or lock them up.