Update on the Black Australorp hens...
During late winter, the shell quality deteriorated rapidly on the Black Australorp eggs. They became gritty, and very fragile. None of the other hens in the same coop, on the same feed, produced eggs of thin shells or uneven texture.
HOWEVER, the egg shell quality returned to normal in a month or six weeks, and has continued just fine.
They lay well!
Of the five hens, I had only one set during the spring season... and she was not as 'dedicated' as a good hen should be.