Recently one of my hens has started laying larger than normal eggs, but, the end of the egg shell is very thin..not the whole egg, just one end. Many times it has a 'swirly' look to the shell, and when I tap it with a finger nail it collapses. I feed layer feed, they get plenty of light, kitchen scraps, and oyster shells are mixed into the feed in the delivery tube. They are about 3 years old. Any ideas???