Migration of Birds

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 subpecies 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 transgulf 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).

Figure 25. Breeding range and migration routes of the Connecticut warbler. From the breeding range in northern United States and southern Canada, it migrates east in the fall to New England, then south along the Atlantic coast to Florida and across the West Indies to winter in South America. In the spring it does not return by the same route but rather completes a loop by migrating northwest across the Allegheny Mountains and the Mississippi Valley (Adapted from Cooke 1915a).

Numerous other loop migrations have been documented throughout the world. In the fall, the short-tailed shearwater, is observed off the west coast of North America as far south as California. At this time the species is on the eastern leg of a tremendous figure-eight circuit around the Pacific Ocean (Fig. 26). The subalpine warbler and red-backed shrikes perform loop migrations between Europe and Africa. Both pass much farther to the east in the spring than in the fall (Moreau 1961). The Arctic loon travels south across inland Russia to southern Europe but returns to its Arctic breeding grounds via the Gulf Stream on the sea because this water is open much earlier in the spring than the inland waterways (Welty 1962).

Figure 26. Migration route of the short-tailed shearwater. An example of an incredibly large loop migration pattern in a pelagic species. Breeding adults return to two islands in Bass Strait during the last part of October after completing a figure-eight circuit of the northern Pacific Ocean (From Serventy 1953).

Dog-legs
Dog-leg migration patterns are characterized by a prominent bend or twist in the route. Studies have shown some of these illogical, out-of-the-way means for connecting wintering and breeding areas have no biological function, but instead, are the result of tradition much like the lineage of crooked streets in Boston can be traced back to old cowpaths (Welty 1962). Many species have extended their range in recent years, but the pioneers continue to retrace the old route from the point of origin even if the new areas are not on the same axis as the earlier route. The old pathways have apparently become implanted as part of the migratory instinct in all members of particular populations even after extending their ranges considerable distances from the original.

Good examples of this crooked traditional path can be seen in the routes taken by Old World species extending their ranges into the New World from Europe and Asia. The European wheatear has extended its range into Greenland and Labrador where the local breeding population has become a separate race. When the Labrador individuals depart from their breeding grounds, they proceed north to Greenland, their ancestral home, then west to Europe and south to Africa, the traditional wintering area for all wheatears. Alaskan breeding wheatears migrate to Africa in the opposite direction via Asia where the Alaskan population presumably originated. Alaskan breeding Arctic and willow warblers and bluethroats also migrate westward into Siberia and then southward on the Asiatic side. Some investigators believe the Arctic tern colonized the New World from Europe because when this bird departs for the south it first crosses the Atlantic to Europe, then moves down the eastern Atlantic coast to Africa and either back across the Atlantic to South America or continues south down past South Africa (Fig. 11). To get to South America from the eastern Arctic, it would be shorter to follow the golden plover's flight path straight down the Atlantic or along the east coast of the United States but the fact that no Arctic terns have been observed in the Caribbean indicates that they do not follow that route.

Figure 27. Migration route and wintering grounds of California gulls banded in northwestern Wyoming. During fall migration, the birds proceed west from the breeding grounds to the Pacific Ocean before turning south to wintering areas in California. A more direct route across Nevada would entail a trip through relatively barren country (After Diem and Condon 1967).

In western United States, California gulls nest in various colonies around Great Salt Lake and Yellowstone Park. Banding records indicate these populations winter along the California coast (Fig. 27). Instead of traveling southwest by the shortest distance to the wintering grounds, they proceed longitudinally down the Snake and Columbia Rivers and reach the coast around Vancouver (Woodbury et al. 1946). Thence they proceed south along the coast to Oregon and California. In the spring the adults return over the same course rather than taking the shorter flight northeast in April across the deserts and mountains; this route would be largely made over a cold and inhospitable country (Oldaker 1961).

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).

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

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).

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.

In the Rocky Mountain region vertical migrations are particularly notable. Chickadees, rosy finches, juncos, pine grosbeaks, Williamson's sapsuckers, and western wood pewees nest at high altitudes and move down to the lower levels to spend the winter. The dark-eyed juncos breeding in the Great Smoky Mountains make a vertical migration, but other members of the species, breeding in flatter areas, make an annual north-south migration of hundreds of miles (Van Tyne and Berger 1959). There is a distinct tendency among the young of mountain-breeding birds to work down to the lower levels as soon as the nesting season is over. The sudden increases among birds in the edges of the foothills are particularly noticeable when cold spells with snow or frost occur at the higher altitudes. In the Dead Sea area of the Middle East, some birds that breed in this extremely hot desert move up into the surrounding cooler hills during the winter (Thomson 1964).

The vertical migrations of some mountain dwelling gallinaceous birds (mountain quail and blue grouse) are quite interesting because the annual journey from breeding to wintering grounds is made on foot. Mountain quail make this downward trek quite early in the fall well before any snows can prevent them from reaching their goal. Blue grouse perform essentially the same journey in reverse. During midwinter, these birds can be found near timberline eating spruce buds protruding above the snow.

These illustrations show that the length and direction of a migration route are adapted to the needs for survival and are met in some cases by a short vertical movement or great latitudinal travels in others.

Pre-migratory Movements
Recent banding studies have demonstrated many migrants, especially young of the year, have a tendency to disperse after fledging. These premigatory movements have also been called "postfledging dispersal," "reverse migration," and "postbreeding north ward migration." Demonstration of this phenomenon is especially important as it relates to locality-faithfulness (Ortstreue), range extension, and gene mixture between populations. These movements cannot be considered as true migrations even though they are repeated annually by the species between breeding grounds and some other area. These movements are generally repeated by the same age class in the population but not the same individuals.

Nevertheless, these regular northward movements are quite striking, especially in herons. The young of some species commonly wander late in the summer and fall for several hundred miles north of the district in which they were hatched. Young little blue herons as well as great and snowy egrets are conspicuous in the East as far north as New England and in the Mississippi Valley to southeastern Kansas and Illinois. Black-crowned night herons banded in a large colony at Barnstable, Massachusetts, have been recaptured the same season northward to Maine and Quebec and westward to New York. In September most of them return to the south.

These movements have been noted in several other species as well. Broley (1947) nicely illustrated this northward movement of bald eagles along the Atlantic coast (Fig.29). Birds banded as nestlings in Florida have been retaken that summer 1,500 miles away in Canada. Van Tyne and Berger (1959) surmised the summer heat of Florida was too great for this eagle, a northern species that has only recently spread into Florida to take advantage of abundant food and nesting sites, which it exploits during the cooler season. Postbreeding northward movements are also shared by wood ducks, yellow-breasted chats, eastern bluebirds, and white pelicans.

A somewhat different type of postbreeding migration is the socalled "molt migration" exhibited by many species of waterfowl (Salomonsen 1968). These birds may travel considerable distances away from their nesting area to traditional molting sites where they spend the flightless period of the eclipse plumage. At such times they may move well into the breeding ranges of other geographic races of their species. These movements may be governed by the availability of food and are counteracted in fall by a directive migratory impulse that carries those birds that attained more northern latitudes after the nesting period, back to their normal wintering homes in the south.

Vagrant Migration
The occasional great invasions beyond the limits of their normal range of certain birds associated with the far North are quite different from migration patterns discussed previously. Classic examples of such invasions in the eastern part of the country are the periodic flights of crossbills. Sometimes these migrations will extend well south into the southern States.

Snowy owls are noted for occasional invasions that have been correlated with periodic declines in lemmings, a primary food resource of northern predators. According to Gross (1947), 24 major invasions occurred between 1833 and 1945. The interval between these varied from 2 to 14 years, but nearly half (11) were at intervals of 4 years. A great flight occurred in the winter of 1926-27 when more than 1,000 records were received from New England alone, but the largest on record was in 1945-46 when the "Snowy Owl Committee" of the American Ornithologists' Union received reports of 13,502 birds, of which 4,443 were reported killed. It extended over the entire width of the continent from Washington and British Columbia to the Atlantic coast and south to Nebraska, Illinois, Indiana, Pennsylvania, and Maryland. One was taken as far south as South Carolina.

Figure 29. Northern recoveries of young bald eagles banded as nestlings in Florida. The birds sometimes "migrate" over 1,500 miles up the coast during their first summer before returning south (From Broley 1947).

In the Rocky Mountain region, great flights of the beautiful Bohemian waxwing are occasionally recorded. The greatest invasion in the history of Colorado ornithology occurred in February 1917, when it was estimated that at least 10,000 were within the corporate limits of the city of Denver. The last previous occurrence of the species in large numbers in that section was in 1908.

Evening grosbeaks likewise are given to more or less wandering journeys, and, curiously enough, in addition to occasional trips south of their regular range, they travel east and west for considerable distances. For example, grosbeaks banded at Sault Ste. Marie, Michigan, have been recaptured on Cape Cod, Massachusetts, and in the following season were back at the banding station. Banding records and museum specimen identifications demonstrate that this east-and-west trip across the northeastern part of the country is sometimes made also by purple finches, red crossbills, and mourning doves.