I've always been told that chickens use rocks in their stomachs to grind up their food. How should I best make this available to them? What size rocks? Are there any particular types that are better than others (pea gravel versus quartz) and how often do they need to replace what they have? Will oyster shell serve this purpose as well as their need for calcium? My chickens free range a couple of times a week for about an hour at a time and there are small rocks in the yard but I've never noticed one pecking at or swallowing a rock. I throw some small (pea sized to about 1/8 pea sized) rocks in their pen every month or so and wonder if this is sufficient or completely wrong. Please share your knowledge!