It's the thick-walled muscular pouch that grinds food in a bird's lower stomach
... n gizzard thick-walled muscular 
pouch below the crop in many birds and ... Gizzard (Anat) The second, or true, 
muscular stomach of birds, in which the food is crushed and ground, after being 
softened in the glandular stomach (crop), or lower part of the esophagus; the 
gigerium.