At some point in the last few days, one of our chickens laid an unusually soft-shelled egg.
I'm not certain when this happened, because I found the egg in the chickens' bedding. Normally, the chickens lay in their nest boxes, but at the moment, Maria Lucas is broody. She's acting all kinds of crazy, and deterring the use of all three nest boxes. So I discovered this particular egg on the floor of the henhouse along with another perfectly normal egg.
The shell of this egg is paper-thin. There must have been a disruption in egg-production. Somehow, less shell was deposited around the egg.
We'll give our girls a bit more calcium in the form of ground-up eggshells, and keep an eye on things.