I think (although I don't know much lol) that when my ducks are experiencing extreme emotions, they kinda move the same. So head bobbing with my three girls means they are really really excited. It can mean good excited, yaaay here comes mom to let us out the hut in the morning, or bad excited, noooo here comes dad and he's gonna snoogle us when we are busy diggin worms. It can also mean they are gettin flirty especially if they are round water but then one of them usually flattens herself out so it becomes quickly clear that they are flirtin! They are all girls and they jus up and flirt with each other. They will climb up on top of each other and go through the motions. (we are gettin a boy duck on loan for a few months over Christmas for them lol)
Tail wagglin either means they are pleased with themselves cos they jus ate somethin tasty, or embarrassed cos they jus fell over sideways into the pool.
lol, so I kinda gotta take in the entire situation when I see them doin somethin cos it sorta can mean good and bad in my house! Their quacks differ though, they cuss up my husband something good, that's how I know it is ticked off bobbing not happy bobbing for him lol!
Sometimes they do it to priss, like they are keeping each other and me in line. Could they be doing this? Do they go into a little trio and wakwakwakwak and bob head to head? Like they are having a little angry meeting? That appears to be prissin and making a check of everythin!
Hope this is some help of how I translate their body language lol!