We have had a few more shellless eggs this week. The problem had clearly affected more than one hen. We increased the amount of calcium in their diet by adding in a teaspoon of poultry spice to their mash. One of the neighbouring allotment holders told me that when he used to keep hens, he roasted the shells until they were brittle and fragile, then turned them into powder using a pestle and mortar which he added to their feed. We are trying that out now - we have a large supply of shells at the moment as we are making lemon curd and I am about to make a load of flans.
The problem however has disappeared. So far today we have had 6 good size eggs, all with shells. Nevertheless, the use of the shells in this way appeals to me as it turns a waste product into something useful and also gets around the regulations that ban the feeding of kitchen waste to hens as the waste is suitable heat treated. I had always been reluctant to feed the hens their egg shells as it could encourage them to eat their own eggs. However, turned into a powder means feeding them something that is far removed in appearance from eggs.
I remain baffled as to how the shellless egg problem started. We give the hens a very good supply of grit. Perhaps some of them felt they should just ignore it.