Oh I love mint!!! YAY I am going to try growing that around the edges of my raised bed-which BTW we start gathering the rocks for this week. I know the chickens will eat the mint because I have purchased it from the store and fed it to them. Any herbs that will deter the chickens?
I was reading another page (and an eHow page) that using river rock gravel helps keep chicken's away from your plants because they don't like walking on it. If you have a large enough space, using rocks instead of mulch could prevent them from going there in the first place. That being said, when a chicken is determined to get something delicious, I've seen them try to get on a crackling fire pit, so discomfort probably means little to them in comparison to a tasty treat, haha!
It's still worth a shot, though. It would certainly stop them from digging up your plants.