Along with food and water, homing pigeons need grit -- or small rocks -- to survive. Because birds do not have teeth or stomachs, they must grind their food in their gizzards using grit in order to digest it. Commercial grit for homing pigeons is available, but you can also make your own fairly easily. (Reference 1)
Place concrete pieces or rocks on hard surface. Smash with sledgehammer until pea-sized pieces remain. If grit is too large, pigeons won't eat it. If it is too small, they will have to eat it constantly. (Reference 1)
Place pea-sized gravel in large bucket.
Add crushed oyster shells in ratio of one part oyster shells to three parts gravel.
Add crushed charcoal in same ratio as oyster shells. Sprinkle mixture with small amount of salt.
Stir mixture until uniform. Provide grit to pigeons in covered tray to avoid contamination from droppings. (Reference 1)