Shell grit is coarsely ground or broken seashells. It is used, among other things, by birds as a source of calcium for eggshell production, (hardening the eggshell), and to aid digestion.
Assisting digestion may be more common with Pigeons and Doves, as they apparently swallow the entire seed, Grit helps the digestive system function well and break down the food as it should. Other uses include protecting plants from slugs or snails and in aquariums.
If birds are able to free-range, they should be able to find enough grit on the ground to peck at to satisfy their digestive system requirements. However, adding shell grit/ground up eggshells to their diet is a great way to keep calcium levels up.
Aids digestion in birds
Made from crushed seashells
Popular mineral & calcium supplement for birds
Added charcoal and an additive which makes the grit more attractive to pigeons and gives it its pinkish colour