I miss the sound of mourning doves, too! Never see them up here in WA, at least not in western WA. And she must have laid another egg when you weren't looking. Can't have twins with an eggshell--no room for expansion! When I used to breed zebra finches, I broke open an unhatched egg and found two little embryos side by side. I mentioned this to my biology professor, who was disappointed that I hadn't preserved the specimen, and explained why they could never have developed. They grew as big as the shell would allow, and then, nowhere to go.