Last weekend Zoe and I saw a pair of ducks on the Kennet that were systematically nodding at each other. The pair consisted of a male and a female mallard so I assume that this was some form of courting ritual. One would bob its head a few times then the other would do it in return. It was quite striking to watch, but we couldn't stay long as we were in a hurry to get home.
I saw what was probably the same pair nodding at each other again on two occasions over the following week. I am fairly sure that the nodding was not just them scooping up and eating the small bit of leaves that float down the river. The scooping is more of a foward and backward motion while the nodding is definitely up and down.