Sladen (1973) has mapped the migration routes of whistling swans, and several dog-leg patterns are apparent in the eastern and western populations ([Fig. 28.]). In the eastern population, a sharp change in direction occurs at their major feeding and resting areas in North Dakota. After the birds arrive from the Arctic breeding grounds, they proceed east-southeast to their wintering grounds on Chesapeake Bay. In the western population, thousands of birds migrate from the Alaskan breeding grounds to the large marshes along Great Salt Lake. Then after a major stopover, this population heads west over the mountains to California.
Figure 28. Distribution and migration routes of whistling swans in North America. Birds from the central arctic head south to North Dakota before proceeding east to Chesapeake Bay, while many Alaskan breeders migrate to Great Salt Lake before turning west to winter in California (After Sladen, 1973).
Pelagic Wandering
Many of the pelagic birds observed off our coasts or at sea appear to be nomadic when they are not breeding. These movements are not necessarily at random because there is usually a seasonal shift in the population, often for great distances and in specific directions, away from the breeding area after completion of the nesting cycle. Also the return from the sea to nesting areas is at a definite time of year. This may not be true migration in the classical sense (Thomson 1964), although it is similar in most respects.
Because of the extensive and often inhospitable habitat of pelagic birds (to human observers at least), observations on their movements are difficult at best and accurate records are few. We do know some of these species have regular routes (e.g., Arctic terns) and specific patterns of migration (e.g., the loop in the short-tailed shearwater). As more knowledge is accumulated on the "nomadic" species, we may actually find they too have regular migration routes based on biological needs.
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).