Miss Lydia posted before I finished typing... Male duck is a drake, a female is a hen or a duck.
Your girl is not odd... I think the head bobs can mean different things. Mine do it most when I'm changing their water... I take it to mean "hurry up and get out of our way!" or "thanks mom"...
Check this out... http://www.majesticwaterfowl.org/artquacks.htm Has sound clips of quacks. But I have to disagree with the freckles on the bill part, because I have a drake that developed freckles.
I kind of agree with you! I think she does it when she gets excited about something...whether it's a good excited or an anxious excited I'm not sure!