We have a small backyard flock of 7 hens (various heavy breeds aged 1-3.) We feed free choice layer pellets (Blue Seal Organic Life), and offer crushed oyster shell and granite grit free choice. Occasionally they get a handful of scratch or leftovers. They free range in the yard all day, too, and I know they are eating a lot of forage. We had some extremely warm (unseasonable and fairly sudden) weather a week or so ago. Nothing else has changed (no diet change, etc.) except that for the last week -since I found the first thin-shelled egg- I have been mixing oyster shell right into their food as well as offering it free choice as usual. So, if anything I'd think they were getting a LOT of calcium. (This bag of oyster shell is a year old, but I don't believe it has a shelf life, does it?) All of a sudden, at least 3 of our hens are laying very thin-shelled eggs. I have found a couple on the droppings board under the roost in the morning, a couple on the floor, and a couple in the nest box, broken and sometimes eaten! This is all very strange. Until now we've had only thick-shelled eggs and everyone except one hen lays routinely in the nest boxes. I looked in the "chicken health handbook" and read about Vitamin D deficiency, that it can sometimes be precipitated by unusually hot weather. So I thought, oh, that must be it! But then I read that TOO MUCH Vitamin D can cause those pimply calcium deposits.... and I know that one hen has been having those deposits for at least 4 months. So--- now I am thoroughly confused as to the cause. And worried, since it's affecting so many of them!!! Today I ground up some eggshells and mixed it with a little chick feed and water and fed it to them as a "treat", hoping that might help some.... but I really want to know what to do next. Anyone?