Grit for pigeons?

A buddy and I just built a pigeon coop and are in the process of trapping feral birds. We have 6 in there so far, room for about 18. Don't laugh too hard, at least we hardly used any "new" material!
Since these birds will need some time to feel "at home" in the coop, we are not letting them out for what will probably be several weeks. In the mean time, I figure they need access to some grit in order to digest the cracked corn we're feeding them. I live on a river beach and could easily keep a cup of sand in the coop for them, but is this suitable? Anyone know?
Since these birds will need some time to feel "at home" in the coop, we are not letting them out for what will probably be several weeks. In the mean time, I figure they need access to some grit in order to digest the cracked corn we're feeding them. I live on a river beach and could easily keep a cup of sand in the coop for them, but is this suitable? Anyone know?