Leap-frogging
When two or more races of the same species occupy different breeding ranges on the same axis as migratory flight, the races breeding the farthest north often winter the farthest south. Thus, a northern race "leap-frogs" over the breeding and wintering range of the southern populations. This has been well documented in the fox sparrow discussed previously ([Fig. 10.]) and is exhibited by races of Canada geese breeding in central Canada as well. One of the smaller races of this goose breeds along the Arctic coast of the Northwest Territories and winters on the Gulf coast of Texas and northeastern Mexico, while a much larger race breeds in the central United States and Canada but winters in the central part of the United States. This leaping over occurs in other species as well, including the bluebird (Pinkowski 1971).