Here's my take. All social animals have a range of body language with subtle variations. Some have more variation than others (consider how much a human can say with facial muscles alone compared to the static face of a duck). But in all cases that I know of, there are "families" of body language that are related to each other.
One "family" is "submissive," "non-threatening," "friendly," "flirty."
It makes sense that they're related because in order to be friendly, you need to demonstrate that you are not a threat. And in order to flirt, you need to demonstrate that you're friendly.
In humans, this is usually smiling, head ducking, peering out of corners or tops of eyes (rather than directly), widening of eyes, and certain types of laughing (which isn't strictly body language, but it's related). We use these signals to tell someone, "It's okay, I'm not going to hurt you," "Please don't hurt me, I'm not a threat," "I like you," and, "Hey baby, let's get it ON!" And everything in between. Which message is intended is usually clear from context and subtleties of body language.
Ducks are the same way, only they are even more limited in the subtleties of what they can convey, so you must depend even more on context.
Head bobbing falls into the "submissive," "non-threatening," "friendly," "flirty" gesture family. So maybe she thinks you're a drake and she wants to mate. Or maybe she's just saying hello, I like you. Or maybe it's "please don't hurt me!" (but probably not). In any event, she is most definitely talking to you and yes... wouldn't it be nice if someone would write a book about this?
