I've recently merged chicks (now laying) with my older hens. Because I've had two flocks I've also ended up with surplus grower feed. In order not to waste this I have mixed layer feed with grower feed. They do have access to oyster shells aside from this. In addition I live on an island so there is access to shells in the soil. All chickens have continued laying as normal except for one chicken who now produces eggs with very brittle shells. First time it happened I couldn't be sure if it wasn't any of the new chickens with a malfunction, but once this happened more than once I got them back on only layer feed again. However, a week or two has passed and she still lays brittle eggs. How long does it take to replenish calcium deposits? Or could there be other reasons this happens? The chicken I suspect is low on the pecking order (though not the lowest) and might just not eat as much as the others. She's also the least bright chicken of the bunch and might not even eat any of the oyster shells. There are 3 feeders for 9 chickens and there is always room at the feeders, so that shouldn't be a problem. How can I help and is there a way to still feed them the layer feed without running into this issue again? The older chickens are a year and a half, the newer just started laying, so age is not an issue. No other apparent health issues. I've started saving egg shells to feed back to them but also might not the rest of the chickens end up with too much calcium?