gizzardnoun [ C ]
/ˈɡɪz.əd//ˈɡɪz.ɚd/a part of bird's stomach that has thick muscles, where food is ground down (= broken into small pieces by pressing or squeezing), especially with very small stones:
Most water birds keep grit, quartz, and other small stones in their muscular gizzard to help grind up food.