Bellrose (1968) identified corridors of southward migrating waterfowl east of the Rocky Mountains and determined, through statistical analyses, the relative abundance of birds in each. He showed major corridors of dabbling duck movements down the Great Plains and Missouri-Mississippi river valleys with minor off shoots at various points from these corridors eastward to the Atlantic coast where they joined equally minor eastern movements from the North (Fig. 13). Bellrose's map of migration corridors for the diving ducks showed heavy traffic similar to that of dabbling species down the Great Plains and relatively heavily used corridors from these central arteries eastward across the Great Lakes area to the Atlantic coast, terminating particularly in the vicinity of Chesapeake Bay. A fairly well-used corridor extends along the Atlantic coast. [Illustration: _Figure 13. Migration corridors used by dabbling ducks east of the Rocky Mountains during their fall migration (After Bellrose 1968)._] [Illustration: _Figure 14. Distribution and migration of Harris' sparrow. This is an example of a narrow migration route through the interior of the country._] With our present knowledge of bird migration it is difficult at best to recognize distinct broad belts of migration down the North American continent encompassing groups of distinct populations or species. It seems that so much intermingling of populations occurs that distinctions between broad "flyway" belts are not discernible. About all we can say for sure now is that birds travel between certain breeding areas in the North and certain wintering areas in the South and that a few heavily traveled corridors used by certain species, and more generalized routes followed by one or more species, have become obvious. Narrow Routes Some species exhibit extremely narrow routes of travel. The red knot and purple sandpiper, for example, are normally found only along the coasts because they are limited on one side by the broad waters of the ocean, and on the other by land and fresh water; neither of these habitats furnish conditions attractive to these species. The Ipswich race of the savannah sparrow likewise has a very restricted migration range. It is known to breed only on tiny Sable Island, Nova Scotia, and it winters from that island south along the Atlantic coast to Georgia. It is rarely more than a quarter of a mile from the outer beach and is entirely at home among the sand dunes with their sparse covering of coarse grass.