what does this duck head bobbing behavior mean?

[micaelasducks]

These ducks are all female, they often all look at each other and bob their heads. For these ducks, what do you think they are saying to each other? They are about 3 months old. Here is a video:

 

[tatswell]

Likely being social. Possibly ready to 'mate'. When my girls do it it is usually followed by what I call alligator- stretching out like a gator, then trying to mate.