The past couple of days I've been feeding them the frozen eggs I'm finding in their nest boxes. I just toss them in boiling water for ten minutes or so, let cool for a bit, then throw the whole thing, including the shell, in my Krups mini grinder and pulse it 4 or 5 times. Wow do they go nuts for that!
If I'm just giving them shells I plop them in boiling water for a few seconds then dry them out in a very low oven for a half hour or so (or just let them sit overnight) then I toss them in the grinder and pulse a few times so there are very small pieces but little powder. Then I just sprinkle them on the ground.