I think feeding crushed eggshells to my hens has improved the quality of their shells. I got two new hens from a friend a few weeks ago and noticed that their eggshells were quite thin, whereas the shells of my hen that I've had for months are very strong. My friend feeds commercial layer pellets, and so do I, but I mix crushed eggshells in with mine. Within 2 weeks, the shells of my new hens had thickened up noticeably. What else could it be besides the eggshells? My girls free-range for only 1 hour a day, so I dont think it's the bugs.