Panting or happy?

[nive]

When my duck sits on my knee it sort of opens and closes it's mouth like it's panting and kind of vibrates. It only dose it when it's sitting in my knee and I'm fussing it. It's there voluntarily and can pop down for a drink freely.
Any ideas why it's doing this?

 

[Amiga]

I have read about this behavior in ducks and it seems harmless, from what I recall.

 

[DisneyDuckies]

My drake does this a lot. I think its hormone related.