A cold rain, mixed with sleet, fell from the pewter sky. We were freezing and huddled against the rain. On the reservoir, more than a dozen ducks ignored the weather: Most of them repeatedly dived into the frigid gray waters.

The ducks were big and brilliant, even in those poor viewing conditions. On the surface, the males were a striking pattern of black and white. The tell-tale long, narrow, red bill was visible, too.

Common mergansers (Mergus merganser) love cold weather.

These large ducks breed from Alaska to the Maritime Provinces and only head south when the lakes up north freeze. Permanent resident populations can be found along the Pacific Coast, the Rockies, and New York/New England. Migrating mergansers winter in the United States from California to New Jersey, including much of the Chesapeake watershed.

Common mergansers are most at home in deep, clear lakes and cold, fast rivers. Although they prefer fresh water, they can be seen in estuaries, bays and along the coast.

They eat fish of all sizes, but they are omnivores. Their diet includes mollusks, frogs, small mammals, other birds and plants.

On their breeding grounds, mergansers typically build their nest in abandoned holes in trees near lakes. They produce one brood a year. The young are born with feathers and eyes open. Young birds receive no help from their parents in securing food. Within 24 hours of hatching, these fluff balls jump out of their nests, tumbling to the earth from some astonishing heights.

The new chicks follow their mother to water where they will start catching insects for food. By day 12, they are capable swimmers and begin to catch fish, which will become their primary food source for life.

Mergansers have long, serrated bills that are ideally suited for grabbing slippery fish and eels. The decidedly unduck-like bill is bright red.

Out on the reservoir, we could see male common mergansers floating low on the water. They have a bright white body, breast and neck. The heads looked black on this dreary day, but they are actually an iridescent, deep forest green. The black eyes are lost in that dark background. The blood-red bill, ending in a sharp “nail,” extends straight out from the head.

On the surface, mergansers show a black arch above their bright white sides. When the bird is flying, one can see that the dark feathers of the wings are limited to the outside half. Large white wing panels with a fine, black, perpendicular line hug the body. The back is also black, while the tail is dark gray.

Females have that red dagger bill, but the rest of their color scheme is muted. The head and neck are chestnut, with a shaggy crest. Females have a white chin patch and white throats. The body and tail are gray. The top wings are also dark except for a white speculum, which is a panel on the end of wing feathers next to the body.

Common mergansers wear their breeding plumage from November to July, so all of the birds on the lake were in their breeding outfits.

At 25 inches long and with a yard-long wing span, these ducks are hard to miss. They weigh about 3.5 pounds, a bit more than mallards.

In spite of their impressive size, mergansers are not popular as game birds. Most duck hunters do not like eating these oily ducks, and only 30,000 of them are taken by hunters nationwide annually. Other ducks are hunted at rates 10 times higher.

Common mergansers need clear water to see their prey and they exist near the top of the food chain. Both factors combine to make this species a good barometer of environmental health. Waters clouded by algae or suspended sediment are avoided because they reduce the ability of these ducks to find fish.

Pesticides and other toxins are bioaccumulative — which means they magnify their impact — in mergansers. High toxin levels can kill or cripple the birds outright. They can also make the ducks infertile by thinning the egg shell, resulting in the unsuccessful incubation of broods.

Seeing the mergansers on the drinking water reservoir is a good sign. The local water utility will have to treat the wastes from these birds before sending potable water to our homes, but the company will be starting from a firm, clean base.

The miserable weather finally chased us back to the car. We were looking for warmth and shelter from the freezing rain. As we climbed into the car I thought about how our needs were so different from those indomitable birds that relish the icy waters.

As I warmed up, I realized our desires weren’t so different after all. The need for clean water and a healthy environment bind us together much more than an early winter squall could ever separate us.