Our Buff Orp has been laying since last July. We have always fed starter/grower, because we have a mixed age flock, but we provide crushed oyster shell free choice. They also free range close to 12 hours per day. About 1.5 weeks ago, I found egg yolk in the laying box. Then nothing for a few days, and now almost every day for the last week, I'm finding very thin shelled eggs, sometimes broken, in the laying box. Today, I waited for her to lay and as soon as she did, I chased her off the nest so it wouldn't be broken to inspect it. It's very thin shelled. I merely had to tap it with my fingernail for it to crack. She did go through a minor molt, and we have a newish rooster who has been mating the hens pretty hard the last month. The only other change has been a different brand of oyster shell. The OS that we used to get was in small bags, and white. This time, about 3 weeks ago, I purchased a larger bag at the local co-op and this stuff is a dark grey...all the other 13 currently laying hens have perfect shells. The problem hen has been laying normally, 5 or 6 times per week and looks otherwise healthy. So...I'd like to hear what kinds of things could be causing this, keeping in mind she's laying at a normal rate.