The hens were playing tricks on us today. They left us half a dozen regular sized eggs, as usual, and one bonus miniature egg.
The mini egg looked like a normal Australorp egg in every respect except that it was tiny.
We brought it up to the house, cracked it open, and found a tiny egg white and a regular-sized but clear yolk inside. It fried up nicely and the toddler pronounced it delicious! Here it is with a normal egg alongside for scale.
These strange miniature eggs are sometimes produced when a hen first starts to lay. Perhaps this means that one of the hens had not been laying yet and now will be, giving us a few more eggs every week.