Diving ducks (in contrast to “puddle ducks” / “dabbling ducks”) actually belong to two separate duck “tribes”, the Mergini (or seaducks with 20 species, for example: long-tailed ducks, harlequins, eiders, scoters, goldeneyes, and buffleheads) and the Aythyini (with 15 species, for example: pochards, canvasbacks, and redheads). Studies have shown that these diving ducks have evolved higher oxidative capacities in their leg muscles, which power underwater movement in most species. Unlike the dabbling ducks whose larger wings allow them to spring into flight directly, diving ducks use their feet running along the water’s surface to augment the power provided by their wings to gain enough lift for flight. The legs of diving ducks are placed farther back on their bodies. This facilitates swimming underwater but limits their agility on land.
Hooded mergansers are my third favorite duck, after wood ducks and harlequin ducks.

Both sexes can raise and lower their “hood”. The drakes have black heads and large white patches as part of the crest; males also have chestnut flanks that contrast with white stripes against other black flight feathers along their backs.

The crests of hooded merganser hens are solid brown and their flanks are brown-gray.

This more subdued plumage provides more camouflage when the hens are guiding their duckling.
These small ducks capture fish, aquatic insects, crabs, and crayfish with their serrated bills. In the Olympia area, they are common at the Swantown Marina and Budd Inlet. There are also a pair or two to be found at the Billie Frank Jr. Nisqually NWR in both the freshwater canals and the estuarine waters. Like the unrelated wood ducks (and like many common mergansers), hooded mergansers nest in holes in trees or in artificial nest boxes.
Goldeneyes are the sister group to the mergansers. Common goldeneyes are quite common in the Salish Sea. Drakes are relatively easy to identify with their white flanks and the round white spot at the base of the bill on the black head.

Barrow’s goldeneyes are also common in the Salish Sea, but they have a crescent-shaped white spot.
Goldeneye hens have brown heads and gray bodies.

Like some mergansers, common goldeneyes are also tree-nesters. They feed on invertebrates and fish primarily.
Buffleheads are a common sight in both freshwater ponds and the marine environment in Western Washington. The body plumage of drakes consists of a bright white breast and a black back. The head has a large white patch at the back of its head on a background of iridescent green / purple feathers.

As with other ducks, the plumage of a bufflehead hen is more muted: more grey on the breast and sides, and with a much smaller white patch and dark, non-iridescent feathers on her head.

During their 12-25s dives, buffleheads search for invertebrates, such as insect larvae, crustaceans, snails and clams, and small fishes. Normally, it seems that buffleheads, even when in small groups, are foraging solitarily as their dives are not tightly synchronized. But several times this fall, I saw groups of a dozen or more buffleheads diving in synchrony along a muddy bank, similar to the coordinated group feeding that one sees by red-breasted mergansers and common mergansers to drive / ambush schooling fish. In fall, buffleheads may eat seeds of pondweeds and bulrushes.
Bufflehead hens nest in tree cavities, especially abandoned nest holes dug by Northern flickers (and artificial nest boxes too). In fact, the breeding range of buffleheads largely coincides with that of the Northern flicker. As one of the smallest diving ducks (and tree nesters), the small opening in a flicker hole excludes other duck species.
To the casual observer, ring-necked ducks appear to be misnamed; they should be named ring-billed ducks. The most prominent aspect of a drake’s appearance is a white ring between the black tip and the gray body of the bill.

But with his neck fully extended (not shown), a drake ring-necked duck will have a ring of cinnamon feathers at the base of the black neck and above the black breast and back feathers, hence the common name. Hens have gray heads, a white ring on her bill, brown breast and sides, and white eye rings.
Ring-billed ducks feed on both aquatic invertebrates and submerged plants. They nest in deep vegetation of freshwater marshes, often on floating vegetation to reduce predation by land-based predators.
The two scaup species, lesser and greater, can be a real challenge to identify. The differences are fairly subtle. But I think that I have images of the drakes of each scaup species. The first, a greater scaup, was taken in the salt water at Tokeland.

Greater scaups tend to be more common than are lesser scaups in the marine environment (but not exclusively). Several aspects of this bird led me to the decision of greater scaup. First (but not visible in this image), the head is rounded. Second, this bird has a large, wide bill with a large black nail at its tip. Finally, the head not very “tall” (distance from eye to top of the head). The only fly in the I.D. is that this purple sheen on the head is typically associated with lesser scaups; however, the green vs. purple sheen may be an unreliable field mark and very dependent on ambient lighting.
A week or so later, I encountered several pairs of lesser scaup at Capitol Lake, Olympia, a freshwater lake (at present…).

In these drakes, you can see a peak at the rear of the head. The head is taller. And the bill is narrower. The drakes were paired up with hens which I assume were also lesser scaups

While visiting Capitol Lake, I also saw two other diving duck species, a large flock of canvasbacks and a solitary redhead drake. These and other ducks were resting 50-75’ offshore, but they were quite suspicious of me with my telephoto lens. I have not encountered either species when visiting other locations in Western Washington, though they are reported to use both marine and freshwater areas. They seem to prefer larger bodies of freshwater. Canvasbacks are quite distinctive with their long, sloping black bills. Drakes have red-brown heads, bright red eyes, a black breast, and white backs and sides. Hens have darker gray heads and breasts and lighter gray sides and backs.

I didn’t notice the redhead drake in the mix (lower right of image above) until I looked at the pictures. The redhead drake is smaller than the canvasback, has a smaller blue bill, and gray sides and back. Mixed in with the canvasbacks were ring-necked ducks and American wigeons.

Our final diving duck is the surf scoter, another member of the Mergini tribe of diving ducks (along with the harlequins, eiders, goldeneyes, and mergansers). These are primarily marine in the winter and are common sights at Tokeland, off Westport, and in the Salish Sea. While the hens have a dark-brown plumage, the drakes are all black with a white patch on the forehead and the back of the neck.

Drakes also have a large sloped bill whose front half is orange and back half is white with a large black dot.
Steve