Gizzard

Gizzard, muscular portion of the digestive system where food is broken up and ground into particles. Gizzards are most common in birds, but they are also found in crocodiles, earthworms, shellfish, and certain insects.

In birds, the gizzard has a tough, horny lining and holds coarse sand and pebbles known as gastroliths that the bird previously swallowed. As the gizzard contracts, the sand and pebbles grind against hard foods such as nuts, seeds, and insects. The action of the gizzard varies with the kind of food the bird eats. It is relatively large and strong in ducks, finches, and other birds that eat grasses and seeds, but it is poorly developed in fruit-eating birds such as parrots.

The gizzard has enabled birds to lose the heavy jaws and teeth that they originally inherited from their reptilian precursors. As such, the gizzard appears to be an evolutionary adaptation that also helps birds fly better by minimizing body weight.