Movements of some of the tubenoses (Order Procellariiformes, that includes albatrosses, fulmars, shearwaters, and petrels) have been correlated with ocean currents, prevailing winds, temperatures, and general water fertility (Kuroda 1957; Shuntov 1968; Fisher and Fisher 1972). Commercial fishermen have long known ocean currents are very important factors in the supply of nutrients, plankton, and forage fish for larger fish. These same foodstuffs often attract pelagic birds as evidenced by the tremendous concentrations that occur off the Peruvian coasts where the upwelling of cold nutrient-bearing water is evident. Kuroda (1957) found some fine correlations between the route of the short-tailed shearwater and ocean currents. Likewise Shuntov (1968) found the migratory routes of albatrosses were over temperate marine waters of high biological productivity. The Laysan albatross was correlated with cold currents, while the black-footed albatross occurred over warm currents. Many Southern Hemisphere pelagic species have been extremely successful in exploiting rich northern waters during the summer; the group is probably the most abundant and widespread in the world (Bourne 1956). 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). Vertical Migration In the effort to find winter quarters furnishing satisfactory living conditions, many North American birds fly hundreds of miles across land and sea. Others, however, are able to attain their objectives merely by moving down the sides of a mountain. In such cases a few hundred feet of altitude corresponds to hundreds of miles of latitude. Movements of this kind, known as "vertical migrations," are found worldwide wherever there are large mountain ranges. Aristotle first mentions vertical migration: "Weakly birds in winter and in frosty weather come down to the plains for warmth, and in summer migrate to the hills for coolness ..." (Dorst 1962). The number of species that can perform this type of migration pattern is obviously limited to those species adapted to breeding in alpine areas.