Some of the most common and widespread of all aquatic birds are the ducks. They breed on all continents except Antarctica, and are noted for their long-distance seasonal migrations along well-established flyways. Many species such as the mallard (Anas platyrhynchos) and northern pintail (Anas acuta) are indicator species of wetlands, waterways, and water courses, but others, such as the Baikal teal (Anas formosa) and Laysan duck (Anas laysanensis) are very limited in numbers and distribution. Most waterfowl are important food resources for native peoples, and revenues from duck hunters promote local economies and fund wildlife refuges.Wildlife enthusiasts value the rich variety of ducks and the brilliant colors of the males, called drakes, of many species.
Ducks are placed in the avian order Anseriformes along with the screamers, swans, and geese. The long-legged screamers lack webbed feet and are placed in their own family, the Anhimidae. The ducks, geese, and swans are gathered in the family Anatidae and are collectively called waterfowl. The anatids are united in having webbed feet, a boat-shaped body, a dense covering of feathers, and a broad and somewhat flattened bill that is variously modified for straining minute surface organisms, gathering shellfish, grazing on aquatic plants, or catching fish. Ducks differ from swans and geese in several ways. They are generally smaller and have shorter necks and legs, which are set well back on the body. They are good swimmers but walk with a peculiar waddling gait on land. Ducks are also distinguished in having scutellate (overlapping) scales on the front of their legs, while geese and swans have netlike or reticulate scaling. Unlike their larger kin, ducks have two molts each year. The first molt occurs after breeding in males and shortly later in females. During this molt, ducks shed their flight feathers and males trade their bright breeding plumage for the plain plumage colors similar to those of females, called an eclipse plumage or hiding plumage. The plain colors of the eclipse plumage camouflages the flightless birds until their new flight feathers grow. The second molt occurs in late summer or early fall and produces the bright breeding plumage of males in preparation for courtship on their wintering grounds. Ecologically, all ducks are birds of aquatic habitats that reach their greatest abundance in the innumerable ponds, shallow lakes, and marshes of the world. Several species are sea ducks that frequent shore habitats, estuaries, and coastal marshes during the nonbreeding season.
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