At this time of year, it's easy to observe Buffleheads (Bucephala albeola) along the shoreline of Bodega Harbor, especially at high tide. This is one of the smallest ducks to visit our area (at 13.5 inches long, it's the smallest duck in North America).
Adult males are boldly patterned with black and white, while females are brown with a smaller white cheek patch. (Young males look similar to females.)
In the right light, you can see purple/blue/green iridescence on the heads of the males:
Adult males have a white stripe running across the entire wing from front to back (see below).
In females this white patch is confined to a smaller square at the trailing edge of the wing (see center bird in next image).
Although they don't nest around here, you may see the males performing courtship displays (e.g., head-bobbing and wing-lifting). Buffleheads nest farther north near small lakes and ponds in boreal forests (primarily in Canada). I remember being surprised when I first learned that Buffleheads are cavity nesters. They use old tree holes excavated by woodpeckers such as Northern Flickers. Most nests are ~3-14 meters above the ground, but apparently there's at least one record of a nest 27 meters (88 feet) high!
This flock was alternating between bouts of feeding and preening. Below two males are flipped on their sides while cleaning their belly feathers. This position reveals their pink feet.