I was just wondering this also I have 3 hens 1 lays white eggs 1 lays light brown long skinny eggs and 1 lays dark brown short fat eggs this morning I got 2 dark brown and 1 white eggs the second brown egg had very thin shell and was cracked the other was fine we normally have thick shells...
We save the shells from our eggs and bake them and crush them up almost to a dust and feed them back to the hens with their feed they go crazy for it. Its a good source of calcium for them