The shoreline of San Francisco Bay has a bunch of ducks. Of the ones I see a lot of, there are two main species. The Mallards are a duck you will see almost anywhere. The others are the Shovelers. As the name suggests, Shovelers like to shovel around in the murk at the bottom of the water to see what they can find. Their bill is shaped to help with rooting around in this mud. The result can be that their heads are no longer the color of the feathers but are, instead, covered in mud.
There is plenty of muddy shallow water along the shore so the ducks will often take off and head to different feeding grounds. It’s fun to try and get them in mid-flight. If things are closer, they will save energy and paddle to their next spot. The best shots are when they come up covered in mud. They look almost surprised by what a mess they are!