a is the proventriculus which oozes the gastric juice
b is the gizzard cut open to show grinding muscles
Birds swallow food and store it in their crop if necessary. The bird's crop is a large sac at the bottom of the esophagus where birds can stuff food in fast. It can also soften hard food with mucus. Once they swallow the food, it goes into the proventriculus, sometimes called the glandular stomach, a tube-like area which produces a large amount of digestive juices. The food then passes through the ventriculus also known as the muscular stomach or gizzard.
The gizzard is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms, some gastropods and some fish. This is where the food is ground up. A lot of grain eating birds swallow small stones, shells, and sand occasionally to break apart hard seeds between a specialized stomach constructed of thick, muscular walls.
No worries about cavities or lost teeth. The grinding stones in the bird's gizzard wear down eventually and pass through the bird but are replaced with new stones the bird swallows.