Did you know that chickens (and other birds) often eat gravel which goes in their crop, along with food, for storage, then passes through their stomach and into their gizzard where the gravel helps to break the food up. In fact, the crops of doves and pigeons produce "crop milk" which enables them to regurgitate their food for feeding nestlings. Other animals that have crops include reptiles, insects, mollusks, and even earthworms.