In North America, the term waterfowl generally refers to ducks, geese, and swans. All waterfowl species are members of the family Anatidae that includes 150 species of ducks, geese, and swans worldwide. There are around 40 North American waterfowl species, including more than 30 duck, 6 geese, and 2 swan species.
Waterfowl are mostly aquatic and are common to most North American freshwater bodies, but also estuaries and coastal waters, with some species of ducks being closely associated with the sea. In North America, the Arctic tundra and grassland areas represent important summer breeding ranges for many waterfowl species, while major wintering ranges include the Central Valley of California, the lower Mississippi valley, the Gulf coasts of the United States and Mexico, the Pacific coast from southern Alaska to Baja California, and the Atlantic coast from Maine to Florida.
All waterfowl have webbed feet, which make them excellent floaters, swimmers, and divers. They are thick-set birds, with a small head, a short tail, and a flattened bill. However, there is a great variation in bill shape and structure that reflect differences in diet. For instance, dabbling ducks have small projections on their bill edges that create a sieve for filtering plants and small aquatic invertebrates from water, mergansers have narrow and hook-shaped bills for catching fish, and eiders have stout, heavy bills for crushing shellfish. Swans and geese are largest in size, with longer necks, than smaller and short-necked ducks. Waterfowl waterproof their feathers by spreading a secretion from a gland located at the base of their tails and are well-insulated from cold by down feathers and a thick layer of fat under the skin. Most male ducks (drakes) are more brightly coloured than female ducks (hens), but in geese and swans, males and females appear similar. Both sexes molt their body feathers and are flightless for one to two months every year. Ducks molt twice, once in winter before breeding when they develop their colourful breeding plumage and once in fall before migration when they return to their duller non-breeding plumage (called eclipse plumage), while swans and geese only undergo a fall molt.
Waterfowl feed in shallow water (dabbling ducks, swans and geese), by diving in deeper water (diving ducks), and on the ground (geese). They have a varied diet, but most feed mainly on plants and some on fish, mollusk, or aquatic insect. There are particular bill adaptations for particular diets, which allow similar species to co-exist in the same habitat while exploiting different food. Generally, dabbling ducks have a broad, flattened, short bill with sieve-like projections on the margins, but bill structure ranges from a probing and picking tool to a surface-straining device sieving for different sized plant and animal material. Bills of swans are designed to tear submerged aquatic plants, while those of geese are designed to clip terrestrial herbaceous vegetation close to the ground. In contrast, diving ducks preying on fish have a long and narrow hooked bill, while sea ducks feeding on shellfish have heavy and robust bills.
All waterfowl species form monogamous pair bonds, lasting until incubation has begun in the case of ducks or indefinitely, and potentially for life, in geese and swans. Females lay all eggs before they start incubating and males are rarely involved in incubation. In general, clutch size tends to be smaller in swans and geese and larger in ducks, and also tends to be larger in low latitude species than in high latitude ones. Once the female starts incubating, she rarely leaves her nest. In some arctic species, females may fast for the whole incubation period, ranging from 40 days in swans to 20 days in some dabbling ducks. All young waterfowl are well-developed when they hatch, covered with down, with eyes open, and enough coordination to begin feeding within hours of hatching. Soon after hatching, the brood of young leaves the nest, often to never return, and remain a tight-knit family group as they follow their mother. In geese and swans, young stay bonded with their mother throughout summer and into migration, while in ducks, young leave their mother after their first flight. Waterfowl are long-distance migrants that travel thousands of kilometers from winter to summer breeding ranges in large flocks. Along their migration route, waterfowl use stopover points or staging areas, like river valleys, major marsh systems, lakes, and some reservoirs, to rest and feed. There are four major flyways along which the north-south migration of waterfowl occurs: 1) the Pacific and Mexico West Coast flyway, 2) the Central and Mexico East Coast flyway, 3) the Mississippi flyway, and 4) the Atlantic flyway. Even though nearly all North American waterfowl are migratory to some degree, some populations in mild climatic regions have become year-round residents.
Johnsgard PA: Waterfowl of North America, Revised Edition. In. Lincoln: University of Nebraska Press; 2010.