Gizzard - Wikipedia

In birds

Birds swallow food and store it in their crop if necessary. Then the food passes into their glandular stomach, also called the proventriculus, which is also sometimes referred to as the true stomach. This is the secretory part of the stomach. Then the food passes into the gizzard (also known as the muscular stomach or ventriculus). The gizzard can grind the food with previously swallowed grit and pass it back to the true stomach, and vice versa. In layman's terms, the gizzard 'chews' the food for the bird because it does not have teeth to chew food the way humans and other mammals do.

By comparison, although in birds the stomach occurs in the digestive tract prior to the gizzard, in grasshoppers the gizzard occurs prior to the stomach, while in earthworms there is only a gizzard, and no stomach.

Koilin lining

In order to protect the muscles of the gizzard, the organ has a multi-layered membrane - or gastric cuticle - made of koilin, a carbohydrate-protein complex (and not keratin as once believed) to protect the muscles.[2] The thickness of this membrane varies with the types of food the animal eats, with diets heavier in grains, seeds, and insects creating thicker membranes than those consisting of fruits, worms, meat, and other softer edibles. In some animals the membrane is slowly worn and replaced over time, while others will discard the worn lining in its entirety periodically.[3]

The lining is critical to the proper functioning of the gizzard, but in some animals it can play an additional role as well. The male hornbill, for example, will fill its gizzard with fruit and then slough off the entire membrane to present it like a 'bag of fruit' to its mate during the nesting season.[3]

Gizzard stones

Some animals that lack teeth will swallow stones or grit to aid in fragmenting hard foods. All birds have gizzards, but not all will swallow stones or grit. Those that do employ the following method of chewing:

A bird swallows small bits of gravel that act as 'teeth' in the gizzard, breaking down hard food such as seeds and thus helping digestion.[4]

These stones are called gizzard stones or gastroliths and usually become round and smooth from the polishing action in the animal's stomach. When too smooth to do their required work, they may be excreted or regurgitated.