The egg shells simply taste better than the oyster shell, which is why they go ballistic for it and don't bother too much with the oyster shell. I use both egg shells and oyster shells, but I do it a little differently. The oyster shell is in a separate container, and they take just what they need of it. I rarely see them digging in to it, but the level goes down little by little so they are taking it as they need. For egg shells, I hard boil older eggs. I let them cool, then put them in a plastic grocery bag and tie it shut. I put that bag inside another one, also tied shut, set it up on the counter, and beat the stuffin' out of it. I smoosh it and moosh it and mash it. I end up with a bag of crushed hard boiled egg mash complete with shell bits, and I take that out to them. They usually get that once a week or so, and they go nuts for it. I like doing it because I like the extra bit of calcium it provides, but mostly because I like them getting all that protein, especially important when they are molting and in the winter.
Now, I know what you're probably thinking....."That'll turn them into egg eaters!" Well, nope, it hasn't and I've been doing that for my chickens for 2 years. What goes out into the run in the form of an egg-and-shell treat bears absolutely no resemblance to what they see and lay in the nests. They love it, they don't know that it's good for them, and they don't associate the mush they get with freshly laid eggs.
