The fact that there are usually many males displaying at once and any of them can carry out the behaviour, means that I've had trouble recording in it a photo previously. By the time I head a whistle, or several, I have missed the behaviour. This week in the park, when I noticed the mallards courting, I decided to focus on a particular male and try and catch him doing it.
The first step of the grunt-whistle sequence looks like the duck is about to dive. Instead, he arches his head forward forcefully, and his bill tip scoops an arc of water droplets toward the female. I managed to get two shots of this stage. The second shows the droplets quite well.
Then is the whistle, while lifting the tail and head (note the bill is a little open).
After that, the drake turns to the female (top shot), and then swims fast with head forward, almost touching the water.
I will try and get a video of the whole sequence in the next few weeks and upload it here.
Wolfgang M. Schleidt & Erhard Oeser (2011) Konrad Lorenz’s use of cinematic film for studying dabbling duck courtship behaviour and the availability of historic film materials. Wildfowl 61: 45–51. Here.