I stand inside a prism of unexpected colors. A wintry sun slips through salmon clouds in a late afternoon sunset. Grasses of honey and gold stand sentinel at my feet. The river, blue, green and gray, slides through patches of shimmering silver and black. Riding low in the water is a jet black duck. He has an imposing orange, black and white bill.

The duck abruptly gathers itself into a compact ball, and then it's gone in a silent dive through frigid December waters. We had come to the banks of the Choptank River looking for the Chesapeake's iconic canvasback and redhead ducks and their cousins, the buffleheads and long-tails. Instead, I find my attention riveted to the surf scoters (Melanitta perspicillata) that have come to the Chesapeake from their Arctic homes.

Unlike the canvasbacks, redheads and mallards, which are nearer to shore, the surf scoters are spread out in loose groupings, interspersed with diving buffleheads and resting gulls.

They are all here because below the water's magical mix of colors lies hidden bounty for the river's long-distance visitors.

The big bill of the male scoter that caught my attention swells significantly at the base, providing the bird with an imposing tool to root through the rocky river bottom when he's searching for his dinner-clams, mussels and other shellfish.

Surf scoters are diving ducks. At 20 inches, they are about average in size for ducks. It is their blocky heads and heavy bills give the sense that they are bigger than they really are.

Males, like the one who's just disappeared beneath the water, weigh about 2 pounds and are slightly larger, on average, than females.

A sharp, black-and-white pattern on the male's head looks crisp in the last good light of the day. His wings, body and tail are all deep black. His heavy, brilliant bill is his most striking characteristic.

The females in the small flock are brownish black. Instead of the male's sharp white spots on the nape and crown, the female has two ill-defined white areas on either side of her head. Her bill is a drab gray with black splotches, and is a bit less imposing than the male's.

Female surf scoters lay five to eight eggs on the ground in a nest that consists of little more than a layer of downy feathers in a small depression. Nests are usually hidden under bushes or in marsh grasses.

While the female is still incubating, the male surf scoter may leave the nest for good. When the chicks hatch, the mothers display equally indifferent parenting skills.

On crowded lakes, young birds will sometimes become mixed up with other broods. They stay with the new family, apparently without causing any undue concern. It's as if the kids went to have dinner with the neighbors and never came back-and no one seems to mind.

With such parental insouciance, it's a good thing the chicks can feed themselves from birth. On freshwater lakes, the ducklings will eat everything from worms and leeches to beetles and mayflies.

One of three species of scoters seen in the United States, the surf scoter is the only one found exclusively in North America. The breeding range for the surf scoter stretches across the freshwater lakes found in the boreal forests and tundra of northern Canada and Alaska.

During the winter, surf scoters leave the ice and snow of the nesting areas. The birds distribute themselves into the ocean waters along the length of both the Pacific and Atlantic coasts of the United States.

Large coastal bays like the Chesapeake also provide the requisite open waters and the shellfish that are essential to the surf scoter's winter diet.

Adult surf scoters, as their name suggests, like to work the waves. They dive just where the breaking surf is roiling with clams, mussels and oysters. Their sturdy bills are well-suited to the task of pushing aside rocks and snatching the hard shellfish, which they eat whole.

Declining levels of oysters and clams may be having a negative impact on scoter populations in the Chesapeake. But here, near the mouth of the Choptank River, there is clearly sufficient food to support a modest population.

While the scoters dive to fill their stomachs, I'm getting my fill, too. The river's kaleidoscope of sparkling hues, the spectacle of the sunset and the rich palette of feathers and bills fills my eyes.

My nourishment is of the visual variety. But today's late afternoon feast is especially wonderful, and I find myself fully satisfied with the day's offerings.