How do you think the shell is broken down into the material used to make egg shells in the first place? For example, my quail exclusively get crushed sea shell for their calcium shell component and grit at the same time. It never comes out in the same form as they eat it. Sand, gravel and crushed egg shell have the exact same effect on gizzards. However I would say in much larger birds like geese and up I probably wouldn't rely on shell exclusively for grit. Being that the quantity of food to grit requirement wouldn't be good enough. But I base that on partial knowledge.