In their natural setting (like a wild bird) the number one food of a chicken is grass. Second will be bugs that eat grass or bugs that eat other bugs. Third will be anything else including mice and other small critters dead or alive. Everything is fair game for a chicken including grubs and worms. Overall a chicken will eat anything that is edible. Notice that the cured seeds of grasses (such as corn, wheat, rye, oats, etc.) are not a normal staple for chickens. That's because those "foods" are seasonal and only readily available in quantity if man raises them, harvests them, stores them, and then hands them out throughout the year. Man invented grain farming and grain as a food source. Consequently grain is an inappropriate food for all animal life