Yes. Egg shells are fine for chickens. Some bake them to insure any potential pathogens are killed. I consider that overkill and simply let them dry.
I do crush them before feeding with the intent being to keep them from associating the shells with eating their own eggs. Not sure it is necessary, but it doesn't hurt.
@Ocho 8 Farm has a good solution when things start to get bad (thin/no shells). Crushed egg shell is fine for ongoing maintenance. If the hen isn't getting enough calcium in her diet, then they pull from their bones and when this source gets low (meaning their bones are weaker), they start laying thin shells and eventually no shelled eggs.
In that case, I've fed my girl a human calcium citrate + vitamin D3 pill. Their egg returned to normal the next day. If you go that way, there are plenty of YouTube videos on how to feed pills to chickens. It's fairly easy.
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Question- What is their base feed? How much/what kind of secondary foods do they get? Depending on the answers, the egg shells may not be sufficient.