What you said is what I expected to be the case.
By the way - I take oyster shell supplements myself.
For my bones, since I don't lay eggs.
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I currently have about one thousand laying hens. I get white, brown, green, and blue eggs from them.
If the hens don’t get enough calcium the egg shells will be thin regardless of the color. The solution is to make oyster shell available to them and then you get good eggshells.
By the way - I take oyster shell supplements myself.
For my bones, since I don't lay eggs.
(-: