This is ancient, but interesting. It refers to "pica" which means a habit or carving for eating non-food items. Like many British poultry references, it distinguishes between soluble and insoluble grit. Locally, oyster shell is the usual grit. I am not sure what you use. It mentions treatment, but I have no idea whether the medications would still be in use: https://www.canadianpoultrymag.com/...grit-and-digestive-disorders-in-poultry-12937