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The species of waterfowl breeding in North America have distribution patterns that collectively reflect the past geologic and ecological histories of this continent. In general, our waterfowl species may be grouped into those that are limited (endemic) to North America, those that are shared between North and South America, and those that are shared with Europe and/or Asia. Of the forty-four species known to breed in continental North America, the resulting grouping of breeding distributions is as follows:
Limited to North America: Snow goose (also on Greenland and Wrangel Island) , Ross goose, Canada goose (also on Greenland), wood duck, American wigeon, black duck, blue-winged teal, redhead, canvasback, ringnecked duck, lesser scaup, Labrador duck (extinct), surf scoter, bufflehead, hooded merganser.
Shared with Eurasia: Trumpeter swan (whooper swan), whistling swan (Bewick swan), white-fronted goose, brant goose, gadwall, green-winged teal, mallard, pintail, shoveler, greater scaup, common eider, king eider, harlequin duck, oldsquaw, black scoter, white-winged scoter, common goldeneye, red-breasted merganser, common merganser.
Shared with South America: Fulvous whistling duck, black-bellied whistling duck, muscovy duck, cinnamon teal, masked duck, ruddy duck.
Shared with Asia only: Emperor goose, spectacled eider, Steller eider (rarely to Norway).
Shared with Europe only: Barrow goldeneye (Iceland and Greenland).
Native to Eurasia, introduced into North America: Mute swan.
It is thus clear that the strongest zoogeographic affinities of our waterfowl are with Europe and Asia, since twenty-three out of the forty-four native North American species have .populations shared with one or both of these areas. Only six species are shared with South America, and, of these, the fulvous whistling duck has a more general tropical distribution that includes Africa and southern Asia. Consequently, it would appear that South America has played only a minor role in providing waterfowl stock for North America, and vice versa. Certainly the great number of waterfowl species shared between the North American and Eurasian landmasses can be largely attributed to Pleistocene and post-Pleistocene history. Ploeger (1968) analyzed the distributions of eighteen species of arctic-breeding Anatidae and concluded that both their present distributions and their described geographic variations could be at· tributed to the physical-geographical situation existing in the Northern Hemisphere during Late Glacial times. Only a minority (38 percent) of these species exhibit noticeable geographic variation, and most of the eighteen have breeding ranges that include both North America and Eurasia. The exceptions are three Eurasian geese (red-breasted, bean, and lesser white-fronted geese), three North American geese (Canada, Ross, and snow geese), and the North Atlantic barnacle goose. It is of interest that these are all geese, a group noted for their strongly traditional wintering and breeding grounds, as opposed to the less tradition-bound ducks.