Oyster shell, not grit. Rollaway nesting boxes are great for this because the birds can't get at the eggs. Also you can make mustard or dish soap filled eggs by blowing the contents out of an egg and filling them with mustard or dish soap. Put the eggs were they can be eaten and the horrible taste tends to turn the birds off of egg eating, at least for a while in my experience... The only surefire solution is starting over with a new flock, but I didn't want to cull any of my birds so I did rollaway nesting boxes (which also stopped my dogs from stealing the eggs) and I've done the mustard filled eggs... There's an article on here that's pretty darn good. I think if you just search "egg eating" in articles you should be able to find it...