How far along is this egg?

[TheSmallestEgg]

Looks like somewhere between day 10-15. I usually deal with muscovies, and they're a bit different though...
As for your question of crosses; khaki campbells x wild mallards will create fertile offspring. kc x muscovies will produce infertile offspring as they're two different duck species.

Ah I see. I hatched moscovys last year, they're definitely odd balls. And honestly, I could be a muscovy egg. There is a muscovy female in there, but I've only seen khakis hanging around this nest box.
Everyone is right, it's very difficult to tell. I think I need to make my guess based on air cell size too. Little guys holding on.