soft shell egg

One of our hens is laying eggs with soft shells. They're like eggs in a plastic bag. Often they break when the next hen gets on the next. Not good for her or the other eggs. As it is just one hen, it is unlikely to be due to a dietary deficiency, and more likely to be due to a defect in the hen. Problem is identifying which hen in order to cull her. Last time this happened we only had three hens. Now we have 11!