fertile eggs?

Not necessarily. My drakes are always chasing around my cayuga hen, and none of her eggs appeared fertile. *sigh* The thing with ducks is that over the generations the females have developed, um, coping mechanisms, to prevent undesirable drakes from depositing their sperm in the correct locations. Their tube is long and twisty with lots of pockets to fool those who are not given admittance willingly. If the hen is willing, you'll usually have fertile eggs, but if not....

I would steal a fresh one to break open and eat, or throw a few in your bator to see if they begin developing. It can't hurt.

Another note, Muscovies are only fertile if both parents are pure bred muscovy. I think this has something to do with the fact that they are actually closer genetically to geese than to ducks (unless I am remembering wrong), so though the eggs will still be fertile, the babies won't be if one parent is another breed. If you aren't raising muscovies, no worry. And if they have muscovies to choose from, they very seldom mate with other breeds, it's just not their way.