A lot of folks, including myself, offer crushed oyster shell that can be bought where you buy your feed. Or, you can feed them their own crushed egg shells which I also do.
It seemed a bit odd to me to feed them their own egg shells, at first. However, it is a really great source of the calcium they need and they seem to prefer them.
Why do I offer both oyster shell & eggshells? Because when they molt they don't lay eggs. When they are broody they don't lay eggs and when they hatch chicks, it usually takes them about five to six weeks to start laying again. When egg production is low, I can use the crushed oyster shell as a substitute.
I will say this, I did a little test to see what my girls would do if I offered both at the same time. Guess what they chose? Their own eggshells! They will eat the oyster shell only if they don't get eggshells after a day or two.
Here's how I give them back their own shells. First, when we use the eggs we rinse them really well and leave them on a paper towel to dry.
Then, I store them in a kitchen counter compost bucket until I need them or the container gets full.
I then put them in a warm oven of 200F and let the oven dry them out for two hours.
After they cool, they are very crunchy and much easier to grind into little pieces with my mortar and pestle. I have a large one so I can put a lot of eggshells in the oven in the bowl and then crunch them up later.
After I crush them to very small pieces, I store them in a re-purposed plastic container that looked better before I put it in the dishwasher and melted it a little. However, it does the job. When the girls need their dish refilled, I pour in the crushed eggshells.
It should be noted that it is very IMPORTANT to crush the eggshells up in very small pieces so that it isn't obvious they are eggshells. You do not want to perpetuate your laying hens starting to peck and then eat the eggs and shells they are laying.
My girls eggshells are very strong and thankfully, my girls all appear very healthy. While the oyster shell isn't really all that expensive, it still helps to save a little on the feed bill by giving them back their own eggshells and who doesn't like to save money?
Finally, I feel better about giving them their own eggshells because I know what I feed them and where it comes from so I know that those healthy eggshells don't have anything in them that might be added. So the next time you eat some of your delicious eggs from your hens, save the shells and give them back.