Chickens -- Calcium

I've been asked to comment in what I use as a source of calcium for my chickens. How could a self sufficient/reliant farm supply this need? 

First, I don't believe that 100% of every resource has to come from the exact acreage of a self sufficient farm. As long as the farm can source the item locally  without undue expense, I believe that is enough. Local sourced, rather than shipped in from outside the region, sits fine with me.

Calcium. Most folks in the USA use ground oyster shells or ground limestone. They simply buy it at the feed store. I could do the same, but why should I? Locally I have a free source of calcium.....coral sand. 

On top of it, the sand is slightly salty from the ocean water. So it becomes a source of salt and trace ocean minerals too. 

I put the sand, right from the beach, into a small bucket which is fastened to the fence so that the birds don't knock it over. I purposely use a small mouthed container so that the hens don't climb in and kick the sand all about. The girls don't eat lots of sand, but over the course of time, they gradually eat away at it. 

My hens also eat small bones in their cooked slop & glop, mice when they can catch them, small birds (a type of dove) when they catch one, plus plenty of green leafy stuff like kale, spinach, and such. So all things considered and since their eggs have strong shells, I think they get enough calcium.