I don't think their shell problem is breed specific or caused by inbreeding, as I've allowed inbreeding of mixed breeds to the same rooster for the past 10 years and basically have has no issues. When I had an instance of thin shells eggs, I never actually observed which hen was laying the bad egg. They are kind of secretive with their laying, but I suspected since I had 10 hens and always only had one bad egg, it was isolated to just one of my hens. Early on, I lost two young hens. One to respiratory (White Rock) and the other to enteroteric disease (RI Red). I isolated them from the main flock, and the rest of the flock didn't get sick. It's pretty easy to identify a sick bird early on because they start getting picked on long before you notice a health issue. Plus sick birds don't lay eggs.
I wouldn't go to extremes trying to fix the problem, because chickens are either hardy or they die. Not worth paying for a vet visit. I would try feeding a high quality layer crumble for a while. Change out their nesting material with some fresh straw, and give them fresh water every day. Try this for a while and let nature take it's course.
I wouldn't go to extremes trying to fix the problem, because chickens are either hardy or they die. Not worth paying for a vet visit. I would try feeding a high quality layer crumble for a while. Change out their nesting material with some fresh straw, and give them fresh water every day. Try this for a while and let nature take it's course.