A cool autumn breeze turned the open water into a corrugated blue carpet and stirred the brown marsh grasses into flickering motion. Close to shore, a small raft of ducks were loafing, heads tucked snugly onto feathered backs. A lone mallard was dabbling at submerged vegetation. A pair of big ducks floated nearby.

In poor lighting conditions and at greater distances, all birds tend to look dark. Many of their subtle colorings and feather patterns are lost without the aid of binoculars or close viewing. Because they are mallard look-alikes in terms of size and silhouette, American black ducks (Anas rubripes) are often misidentified by casual viewers as female mallards. Like mallards, they are big; nearly 2 feet long from bill to tail with a wingspan that approaches 3 feet. They weigh about 2.5 pounds.

The autumn sun was low in the sky. The lighting wasn’t particularly good, but my binoculars compensated, and I readily distinguished the pair of black ducks from their mallard relatives.

In general, male and female black ducks look alike. They have a blackish/brown body. Their faces and necks are a pale tan. They have a dark cap and a distinctive dark eye stripe.

Viewed from underneath when they are in flight, black ducks show a dark body and ivory wing linings. Like mallards, they have a purple speculum, which is seen as a patch on the folded wing of a resting bird.

There are a few minor differences between the sexes. Male bills are yellow while the female bill ranges from mottled orange and black to blue-gray. Female black ducks are somewhat paler overall than males

Like mallards, American black ducks are dabblers. That is, they eat food from the surface of the water or go “bottoms up” to reach underwater grasses, tubers and seeds. In addition to their diet of wetland vegetation, black ducks sometimes visit agricultural fields to eat seeds and grains.

Black ducks are year-round residents of the Chesapeake. Their permanent range is primarily in the northeastern region of the United States. During the summer, black ducks disperse north to Ontario, Quebec and the Maritime Provinces. Those northern individuals will soon be heading south, with the peak fall migration taking place in early November.

During the winter months, black ducks fan out across the southeastern United States and as far west as Missouri.

Although they cover a broad area, they are most at home in coastal salt marshes. Not surprisingly, then, the wetlands of the Chesapeake and its tidal rivers are vital to these large ducks.

Black ducks were once the most abundant dabbling duck in eastern North America. Hunters prized them, and during the last century, they made up the largest part of the Chesapeake Bay’s waterfowl harvest.

Scientists are paying close attention to American black ducks because their numbers are in serious decline. By 2007–09, the Bay population of pure black ducks was down to 37,000 birds.

They are a “Focal Species” on the U.S. Fish and Wildlife Service’s “Birds of Management Concern.” The listing means that the birds require additional conservation attention and resources to help bring them back to a healthy population.

The new Chesapeake Bay Agreement, signed on June 16, contains a numeric goal for waterfowl for the first time: a wintering population of 100,000 American black ducks.

According to the USFWS, black ducks can live as long as 26 years in the wild, although they seldom get the chance to do so. Like most ducks, black ducks have a very high mortality rate. A majority of them will die before they reach their fourth year, victims of nest predation, disease and a host of other causes.

Mallards have been introduced to many parks, ponds and lakes. They are interbreeding with black ducks and displacing them in many former strongholds. Hunting pressure plays a role in the species decline, but wildlife managers are lowering bag limits and open seasons to reduce pressure. The biggest culprit of all is the loss of suitable habitat.

I continued silently watching a single pair of black ducks and a small raft of mallards. The black ducks might be easily overlooked, just as their declining numbers may not register with most people in the watershed.

On this quiet autumn day, I sense the shortening days of year’s end and paused to consider the health of the black duck species as well as my own. Do these shortening days presage my own future? Is this the autumn of the black duck species?

As humans, we have the capacity to stem the slow decline of these unassuming ducks. Instead of a dark omen, autumn can become a cyclical moment that simply leads to a vibrant spring. With equal measures of hope and concern, I realize that the choice is ours.