If your egg shells are hard, they are getting enough calcium. The layer contains some, the table scraps probably don't have much, they get calcium from the hard shelled bugs they eat, and they may get calcium from the rocks and pebbles they eat as grit when they free range if you are in limestone country. It does not hurt to offer some on the side in a bowl. I don't and my eggs are fine, but if there is any question as to the quality of your egg shells, I'd suggest offering it on the side. They do need sufficient calcium. I would not mix it with their food since I believe forcing them to eat too much of anything is probably not good for them.