Ducks are members of the family Anatidae, which also includes geese and swans. The subfamily of "true ducks" can be divided into at least seven groups (or "tribes"), which change as scientists learn more about the evolution and taxonomy of ducks (and about which there is still disagreement among experts).

The whistling ducks, otherwise known as tree ducks, comprise a separate subfamily. They generally resemble small geese or large true ducks and, in fact, are probably more closely related to geese and swans than they are to true ducks. Four of the true duck subfamilies are found in North America: dabbling or puddle ducks, diving or bay ducks, sea ducks and mergansers, and stiff-tails.

Diet

Most ducks are omnivorous, but some are primarily herbivores and others (such as sea ducks and mergansers) are mostly carnivorous. Plants eaten include those commonly available in wetlands such as seeds, aquatic grasses, and submerged pond weeds. Animals eaten include invertebrates, fish eggs, and fish. Many duck species switch from a diet of mostly plants to a diet of invertebrates just prior to breeding, which may help them acquire sufficient energy for reproduction. Young ducks eat primarily invertebrates.

Habitat

Most ducks breed near aquatic habitats such as wetlands. Some species migrate to high-latitude (arctic) wetlands, while others make use of lower-latitude wetlands. Because they are subject to cyclic flooding and drying cycles, the lower-latitude wetlands are highly variable in their suitability for nesting. For example, the prairie pothole region of the Great Plains provides nesting habitat for many ducks: Dabbling ducks typically nest along wetland edges in this region, while canvasbacks, ruddy ducks, and others nest in the aquatic vegetation within potholes. At the moment, however, the prairie pothole region is experiencing a dry spell, which is probably cutting down on reproduction numbers.

Mergansers and goldeneyes breed along lakes and rivers in wooded areas farther north, sometimes laying their clutches of eggs in tree cavities or in holes in rock cliffs. Other species—such as eiders and northern pintails—breed in even more northerly tundra regions, alongside snow geese and tundra swans. American black ducks nest primarily on the ground, but occasionally they may choose tree cavities in the marshes and woods of northeast North America. The true tree-nesters include the wood duck, American goldeneye, Barrow's goldeneye, hooded merganser, and bufflehead.

Migration

Not all ducks migrate each year. But some do make remarkably long migrations from, for example, breeding grounds in Canada to wintering habitat in the southern United States, Mexico, and Venezuela. Sea ducks generally nest inland—some very close to an ocean and others as far inland as the northern prairies—and spend their winters on the ocean or in bays.

Some ducks use their precise navigational ability to return to the same breeding and wintering grounds every year. But others respond to year-to-year variations in habitat suitability (cased by variations in precipitation or severe storms) and choose new wintering or breeding grounds each time they migrate. Although the U.S. government manages waterfowl species on the basis of four north/south "flyways," ducks and other waterfowl often do not migrate along a straight north/south route. They may cross through two or more flyways as they make their way north or south.

Coloration

True ducks (with the exception black ducks) often display striking differences in coloration according to sex: The males are brightly colored, while the females generally sport drab brown or gray feathers. The bright coloration of males may be one way by which they influence a female to mate; coloration, for example, may connote their genetic quality or health. Females, on the other hand, would not benefit from bright coloration, which can attract nest predators. The mallard may be the most well-known example of sex differences in coloration; however, the wood duck—especially the male—is the most colorful duck in North America.

Immature ducks of both sexes often bear duller coloration than that of adults. Adult ducks generally molt, transitioning between breeding plumage (the more striking coloration in males of most species) to non-breeding plumage (also called "eclipse" or "hiding" plumage). Female plumage generally changes less dramatically. Because plumage varies with sex, age, and season, birders may find identifying duck species challenging.

Courtship and Reproduction

Whistling ducks (like swans and geese) form long-lasting, monogamous pair bonds; often a male-female pair will remain together for life, sharing parental duties each breeding season. However, most true ducks, though technically considered monogamous, form new pair bonds each year. This pairing off may begin as early as August, at which time pairs of many species begin to engage in elaborate courtship displays. These displays involve movements and counter-movements that appear to be exaggerated preening, head bobbing, and feeding behaviors, as well as particular calls and postures—such as holding both head and tail up—that are unique to courtship. Sometimes these displays start in a social context that includes a large pool of potential mates.

Once paired, the male of some duck species will defend both a territory and the female from other males, while the female finds a suitable nest site and lays her clutch of eggs. Despite the sometimes fierce watch kept by the male, the female may mate with other males. Genetic analyses now available to scientists have revealed that this phenomenon occurs in many birds who were once considered strictly monogamous. After early incubation, the bond between the parents weakens, and the male of many species will abandon the female, leaving her to defend the eggs from nest predators such as skunks and foxes.

Ducklings imprint on their mother, a process that begins with the mother and ducklings exchanging low calls before the ducklings have even hatched. Almost immediately after hatching, most ducklings will follow the mothers into nearby water. The newly hatched ducklings are covered with a dense, insulating down that traps air, making them buoyant.

The young are able to forage immediately, but they still rely on the mother to defend them from predators. When necessary, she will call them to her with a "contact call." Maternal care in most species lasts until the young are nearly able to fly, but the duration of parental care varies across species. In ruddy ducks, for example, the mother watches over her brood for only a few days; in some sea ducks, she will care for them for a few weeks, after which the young from various broods form crèches attended by one or more adults.

Populations

Some ducks—the spectacled eider and Steller's eider—are listed as threatened under the Endangered Species Act (ESA). Other duck species (for example, the pintail, black duck, canvasback, and scaup) have shown significant population declines, though they are not listed. The difficulty in getting duck species listed under the ESA is that most of what is known about duck populations is based on the abundant mallard. Many other duck species are not as easy to study or survey extensively; therefore, some conservationists are also concerned that several little-known species of sea ducks, as well as wood ducks and redheads, may be disappearing without the scientific community taking note.