I have pound pups for guardians.  Seriously!  Shadow is a bassett mix, that is about 16yrs old.  He is fine with the chickens as long as they don't come close to me.  Then he chases them away.  Magenta is a beagle.  She is 8 yrs old.  She did kill a 6-8 wk old chicken.  Since then, I've held her down, called the chickens over, and got one to jump on her belly.  It pecked her, though that wasn't my intent.  She is scared to death of a chicken now.  Lulu is a 19 month old mini dachie.  She has been pecked by the chickens more times than I want to know about.  They love pecking her tail!  She goes in the hen house with me, and is out with them most of the day.  Some of the hens play tag with her, or rather, chase her around, and Lulu isn't smart enough to know they want to eat her!!!  
I honestly believe that the predators smell that we have dogs.  Shadow is only male dog, and he marks all around the coop and run.  All three dogs are inside dogs though.  At least twice a night, I let them out for potty breaks.  They always circle the coop, and run and bark away.  I'm hoping it is at least keeping things away.  The coop is a wooden shed, that we converted.  The pop door into the run is closed every night.  The people door has two locks on it. Metal tin roof, so I don't think anything can get through that.  The chickens roost in the rafters, so are a good 6-7 ft in the air at night.
During the day the chickens are free ranging.  I'm out there several times a day, and the dogs go out with me.  I don't 100% trust that Shadow wouldn't kill one if he felt threatened by them.  Magenta I use to worry about, but I know she is so afraid that she won't even walk past them to get back in the house!!