I see lots of the drakes do it quite often. And they make that weird grunting type sound when they do it. I mostly notice it directed toward a hen as a mating thing, or so I thought. One of the appleyard drakes did it several times before I started videoing (which is actually why I decided to video). But they didn't do any mating afterward, so I'm honestly not sure.
They are beautiful ducks!
It is a mating dance. It doesn't mean they will mate after it, but it is them showing off their stuff. There was a post about this a while back in which Isaac had mentioned that it was a mating display. There is another meeting display too, but I can't remember the name of it...it is something like head tail flick. It is where they stretch themself up and flick their tail.
Wow! Absolutely stunning ducks and your Muscovy are amazingly gorgeous! So much beauty
You've got talent in the field of photography! 
Sure! After all, I guess I am a duck addict. Lol@Gizmo Carp would you like to come join us over here?
Those are beautiful ducks! What camera did you use to take those shots?
