Are your chickens eating their own eggs? Or maybe their eggs just look abnormal or the shells are thinner then their supposed to be? Well then I have a great method that I've been using for my hens that I'd like to share!
What I've been doing is using the eggs and saving the egg shells for whenever the plate that I had them on was full. Once I have the eggs I would crush them up as much as I could and I would mix it in my hens feed.
Although they do most likely get a lot of calcium from bugs that they scratch for or even just from the bag of feed, it's still a good precaution to put some extra calcium, believe it or not from their own egg shells!
The reason why, is because your hens produce most of their calcium that they get from their food. That's why giving them egg shells back is ok because it will help their eggs get stronger for the next time they lay. This will also help your hens to stop eating their own eggs if they have that problem. The more hard the eggs are the less likely they'll peck on them and figure out that it tastes good.
Even if you don't worry about the eggs to much because they already look pretty healthy, its still always good to give them more calcium for just their body. A hen can never have to much calcium! Hope this helped some fellow chicken owners out their!