I suppose if I thought it was a problem I'd drag out the metal detector and go over things myself. However mine are running around in junk piles full of all sorts of materials, fallen down buildings, leftover nails after the wood rotted off them, stacks of bricks, various rusted metal containers and sheets that just crumble to bits.... I haven't lost one to anything except predators and my hatchet. Maybe I just have extra smart chickens when it comes to food. They will also walk past styrofoam containers without touching them. Or maybe they just have plenty of better things to do. With all the brush, that junk pile and fallen down buildings covered in weeds and blackberry bushes gone wild, and then 40acres of crp (land set aside and planted to feed wild animals like pheasants) to wander maybe mine just don't find a need to see if the nonedibles are potentially edible.