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. 大多数水鸟都会在肌肉发达的砂囊中保留砂砾、石英和其他小石子，以帮助研磨食物。