If the chickens don't feel the need to eat the oyster shell, they won't. But if they want to, they will. Same with the grit. Mine eat little oyster shell or grit in months when free range materials are plentiful and good to eat. During winter months, they eat a lot of grit and a lot of oyster shell. They do have a complete layer feed. I keep mine available in two little bowls along a wall in the coop (in back in the picture).
