It is not something they can prove 100%! We divide matter up into 3 categories - animal, vegetable and mineral. Animal includes all critters that can move on their own- this includes oysters (we use oyster shell for grit and extra calcium), insects and other critters. If a chicken breaks an egg they usually will eat it. If they catch a mouse in their travels they will rarely pass it up as a meal! The more free range a hen is, the more likely she will have animal products in her diet! The Federal Govt. forbids PROTEIN PRODUCTS OF RUMINANTS (cow, sheep, goat) to be included in chicken feeds (not ALL animal byproducts)[title 21 CFR589.2000]