Bird nest from mom's yard

My parents have a bird, a wren, they believe, that tirelessly comes back every year and builds a new nest under the lid of their gas tank. The eggs have long since hatched and the birds gone from the nest, but the beautiful nest remains as well as one lonely egg that never hatched. The pics are of the nest and also we opened up the nest to see what we could see. A head was slightly recognizable and we could actually see some very small feathers.