The term waterfowl, at least as it is applied in North America, is generally restricted to the ducks, geese, and swans of the bird family Anatidae. About 140 species of this group of swimming and diving birds have survived throughout the world to the present day, and four more have become extinct during historical times. Many more species have existed in the past; the fossil record of this family extends back roughly fifty million years to very early Cenozoic times, although very little is known of the actual appearance and structure of the earliest form of waterfowl. Presumably these ancestral birds were semiaquatic, perhaps much like the modern-day species of screamers (Anhimidae), which together with the true waterfowl make up the order Anseriformes. These in turn seem to have been derived from land-adapted and fowl-like birds that later diversified into such groups as pheasants, quails, partridges, turkeys, and other "gallinaceous" species.
In part because of their common evolutionary ancestry, waterfowl and the upland, or gallinaceous, birds have certain similarities in their biology that are more fundamental than the obvious differences in their adaptations to aquatic versus terrestrial habitats. One of the most significant of these common attributes is the rather advanced, or precocial, state in which the young are invariably hatched. This implies that they are well covered with down and thus can better maintain a steady body temperature than can birds hatched naked or nearly so. They also are hatched with their eyes open, and they are sufficiently coordinated so that they can begin feeding on their own in a day or less of leaving the egg. They have a variety of calls and can respond quickly and effectively to calls of their parents that may help to keep the family together and safe from danger. They typically remain together as a cohesive "brood" during the period between hatching and initial flight, or fledging, and during this time they undergo the socialization processes that may be important later in pair formation at the time of sexual maturity. They also learn the local topography and, especially in the case of females, the landmarks necessary to allow the birds to "home" to their natal area at the time of initial nesting.
Unlike most upland game birds, nearly all North American waterfowl are migratory to some degree, and although the timing and general compass-direction tendencies for movement may be innately transmitted from generation to generation, a considerable part of the specific aspects of migration is dependent on a transmission of migratory "traditions" from the older birds to the younger ones by direct experience. This flexibility in migratory behavior accounts for the surprisingly rapid shifts in migratory pathways and stopover points that waterfowl are able to make when major environmental changes occur, such as the establishment of bird refuges, the filling of impoundments, and the like. On the other hand, this adaptability also may cause an area to be "burned out" of its waterfowl use, when disturbance or excessive mortality disrupts the traditional use of an area. This capability for human manipulation of usage by waterfowl in their migratory or wintering areas poses enormous problems for wildlife biologists, who must choose carefully between the desirability of providing safe and attractive areas for use by large numbers of birds and the potential dangers imposed by such congregations: disease' or parasite transmission, crop depredations on nearby private lands, and the encouragement of unrestricted or uncontrolled hunting in areas adjacent to the controlled-usage areas. Interstate and international politics may even become involved, in view of the great economic significance of waterfowl hunting in certain parts of North America.
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