I think the egg was from one of our 2-year-olds. We do have one autumn duckling- my husband and me have a bet outstanding if it's a duck or a drake, we've found/observed no conclusive evidence either way yet. That would be great news...
Stupid question, though: how would blood get inside the shell, if it's caused by straining during the lay? I'll have a look if I can find the other topics, thanks for mentioning that!