Loops
Many species do not return north in the spring over the same route they used in the fall; rather, they fly around an enormous loop or ellipse. Cook (1915a) considered food as the primary factor in determining the course birds took between winter and summer ranges. Individuals that returned by the same route and did not find sufficient food for their needs at that time were eliminated from the population, and only progeny from individuals that took a different course with sufficient food lived to build the tradition of a loop migration. Other investigators consider prevailing winds a major factor in the evolution of loop migration. Whatever the reason may be, it has most likely evolved separately in each species to satisfy its particular needs, and the fact that this pattern occurs all over the world in completely unrelated species is a good illustration of convergent evolution.
The annual flight of adult golden plovers is so unusual, it will be given in some detail. The species is observed by hundreds of bird watchers every year and it well illustrates loop migration ([Fig. 24.]).
Figure 24. Distribution and migration of the American golden plover. Adults of the eastern subspecies migrate across northeastern Canada and then by a nonstop flight reach South America. In spring they return by way of the Mississippi Valley. Their entire route, therefore, is in the form of a great ellipse with a major axis of 8,000 miles and a minor axis of about 2,000 miles. The western subspecies migrates across the Pacific Ocean to various localities including the Hawaiian and Marquesas islands and the Low Archipelago.
In the fall, the birds fatten on the multitude of berries along the coasts of Labrador and Nova Scotia, then depart south over the Atlantic Ocean to South America. After reaching the South American coast the birds make a short stop, then continue overland to the pampas of Argentina, where they remain from September to March. When these golden plovers leave their winter quarters they cross northwestern South America and the Gulf of Mexico to reach the North American mainland on the coasts of Texas and Louisiana. Thence they proceed slowly up the Mississippi Valley and, by the early part of June, are again on their breeding grounds, having performed a round-trip journey in the form of an enormous ellipse with the minor axis about 2,000 miles and the major axis 8,000 miles stretching from the Arctic tundra to the pampas of Argentina. The older birds may be accompanied by some of the young, but most of the immature birds leave their natal grounds late in summer and move southward through the interior of the country, returning in spring over essentially the same course. The oceanic route is therefore used chiefly by adult birds.
A return by the oceanic route in the spring could be fatal. The maritime climate in the Northeast results in foggy conditions along the coast and the frozen soil would offer few rewards for the weary travelers. By traveling up the middle of the continent, a much better food supply is assured (Welty 1962).
Several North American warblers including the Connecticut warbler ([Fig. 25.]) and the western race of the palm warbler have been found to follow circuitous migration routes. The Connecticut warbler is not observed or banded on the East coast in spring, but it is recorded farther inland during the season. Thus this warbler proceeds down the East coast in the fall and up the interior of the continent in the spring. Similarly, the western race of the palm warbler moves from its breeding grounds directly east to the Appalachian Mountains before turning south along the coast. Television tower kills in northern Florida indicate the population is very concentrated here at this time of year. In the spring this race also proceeds north through the interior. Graber (1968) points out that the eastern race of the palm warbler also proceeds south along the coast in the fall and poses this question: "does the western population of this species intentionally move toward the ancestral range, or is the fall flight direction merely a consequence of the temperate zone westerly circulation?"
Graber concluded from radar observations that the disparity in seasonal flight directions of many migrants was a positive response of migrants to favorable wind directions at that time of year. The east-oriented trans-gulf migrants followed an elliptical migration because postfrontal air flow in the fall at latitude 40° N is northwesterly, and, in the spring southerly; whereas winds over the Gulf of Mexico are consistently easterly or southeasterly. Therefore, transgulf migrants returning north in the spring would be moved westward across the Gulf unless they compensated for wind drift. Observers were not aware of high-altitude drift before radar (Bellrose and Graber 1963).
