The only problem with what B's said above is, if they free range, the chickens don't know the difference between regular lawn soil, where you have your garden planted at etc, and if they find a worm or two, will CONTINUE to dig. You may find foot deep holes, and if your strawberry or squash or blueberry vines happened to be at the edge of this excavation, well, RIP. If you are using them to 'aerate' your lawn, you will want to move where they are sitting every few days at most, you really don't want to leave them there long enough ie a week or so to tear it down to bare dirt and out by the roots, which I am sure you have seen by now, they WILL.
Aaron