Oyster Shell Grit - Fine
Birds, of course, don't have teeth. Although parrots and some other birds "pre-process" their food by chewing, food is further broken down and processed in the bird's ventriculus -- popularly known as the gizzard.
The ventriculus is a specialized section of the gastrointestinal tract, tough and muscular enough to grind up food. Small, indigestible items -- grit -- remain in the ventriculus, and further aid in the grinding and processing of the food. The finer the food is ground, the more readily it will be digested, and the more nutritional value a bird will gain from it.
A great source of Calcium.