You could try river stones, about the size of a lime or so. Chickens won't dig in these, for obvious reasons Ouch!. They work well with grass too, but keep in mind that they are hard to dig in, a huge pain in the butt when trying to add more plants. So make sure you have completed your garden...
Are you losing the war against protecting your garden against your backyard free range chickens?
If you have had anything like I have these past summers, you have chickens taking shortcuts through the garden and trampling your plants in the process, digging huge dirt holes, and breaking stems...