Could you have a drake and the drake feather was molted out? lets see if we can get someone else to look at the egg pic.. Do you have an idea who miight be the drake if so what about a loose tie wrap around one leg just so when you see the hop on top you can tell who is who. Have you been close when they are mating you can not miss a waterfowls man part. If your looking for it.
@WVduckchick just to tag a few.