I read that you could recycle egg shells and feed them back to your chickens. So I tried that the other day.
The important thing is that the eggs should not be recognizable as eggs. So I washed and air dried the eggs, then smashed them into small pieces. A month's worth of egg shells didn't look like much when I was finished. They were gone in a couple of days.
Egg shells contain calcium which the chickens need to produce an egg. I usually provide oyster shell for the chickens as well.