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The pothole areas within the glaciated, northern prairies and adjoining parklands of south-central Canada and north-central United States are the principal breeding grounds for many species of North American ducks. Millions of pairs, including dabbling ducks (Anatinae) and diving ducks (Aythyinae and Oxyurinae), resort here for nesting each year. These populations are characterized by instability. In response to the variable climatic conditions of this region and their effect on wetland habitats, the numbers of breeding pairs often fluctuate greatly from year to year. In addition, regional populations are influenced by man. Factors that are especially important in this regard include the modification or destruction of wetlands due to intensive land-use practices, and variations in annual hunting pressure along the migration routes and wintering grounds.