The Chicken class instructor (from now on called the chicken guy) said that chickens are like little bulldozers and that they can tear up a garden in record time. I was dubious about the second claim, but count me in as a believer! They love to move a bunch of dirt and then look for bugs, and will repeat this ad infinitum. It is so obviously a instinctual behaviour.
So far they have stayed out of my garden (maybe because it only contains garlic right now). There is nothing else in the backyard they can harm, so they have the run of the place (supervised in case of predators). A Louisville slugger is my weapon of choice (we have big dogs in the park).