I save all of my eggshells as well (unless it's just ONE shell for a quick breakfast sandwhich or something). I put them all on a microwave safe plate and put them in just as is with a little white still in them, membrane, and so forth on HIGH for 4 to 5 minutes. The eggwhite remnants and membrane will char just a little bit, and it does have sort of a funky odor while it cooks. When it's done, the eggshells are extra crispy and break easily by hand (but be sure to let them cool first, they're EXTREMELY HOT when they first come out of the microwave).
I crunch them up by hand enough to put them into my small food processor, then I grind them into a near powder. This I put back onto the chicken feed.
Ashes to Ashes, Dust to Dust, and Calcium Carbonate to Calcium Carbonate -- it's all good and does the job for the girls.
No reason not to give them back to your ducks as well as to your chickens.
Finally, another good use for the powdered eggshells-- a couple of tablespoons mixed into the soil at the base of a tomato or pepper plant will help a lot to prevent blossom end rot of the fruit if you also keep the soil moist (BER is a syndrome caused either by inadequate soil calcium, dry conditions not allowing the plant to take up soil calcium, or a combination of the two factors).