I use a product called Calciboost, made by The Birdcare Company. I started using it a long time ago, for a CAG with major phobic issues. It had such a dramatic effect on him, I continued its use for all my birds. I only put a drop per 2 oz water container once per week, so a little goes a long way! I have been consistently very happy with the product. I give it to all my rescues, and if you use it as prescribed, the results are almost always dramatic. I used to sell it through my website, and consistently got positive comments about it. (I stopped selling supplements altogether, as I got tired of dealing with merchandise that has expiration dates, but I still LOVE The Birdcare Co line!)
I also have chickens, and I get a couple of dozen eggs per week, so I save the eggshells and grind them up in a mortar and also add that to their seed mix. Because it is such a fine grind, I can't really say whether they eat it or not.