Gizzard, a digestive organ of birds and certain other animals, such as earthworms, grasshoppers, and crayfish. A bird's gizzard has a thick, muscular wall and often contains gravel eaten by the bird. The food first passes to the crop, or gullet pouch, where it is moistened, then to the proventriculus, or glandular stomach, where it is mixed with mucus and digestive juices. Preparation for digestion is completed in the gizzard, where the food mass is ground by the action of the muscular walls and gravel. In smaller animals, the food usually passes directly from crop to gizzard.The gizzard is a muscular organ of the digestive system of birds and other animals.