While certain flight directions are consistently followed by migratory birds, it is well to remember that the term "migration route" is a generalization, a concept referring to the general movements of a species, rather than an exact course followed by individual birds or a path followed by a species characterized by specific geographic or ecological boundaries. Even the records of banded birds usually show no more than the places of banding and recovery, and the details of the route actually traversed between the two points is interpolated. In determining migration routes, one must also constantly guard against the false assumption that localities with many grounded migrants are on the main path of migration and localities where no migrants are observed are off the main path.

There is also considerable variation in the routes chosen by different species. Differences in distance traveled, time of starting, speed of flight, latitudes of breeding and wintering grounds, all contribute to this great variation of migration routes among species. For example, waterfowl banding data not only indicate species differences, but also indicate considerable diversity in direction of movement by different breeding populations within a species as well as between individuals in the same population. Nevertheless, there are certain factors that serve to guide individuals or groups of individuals along more or less regular paths, and it is possible to define such lines of migration for many species.

Flyways and Corridors

The notion of bird populations being confined to four fairly definite and distinct migration "flyways" is probably most applicable to those birds that migrate in family groups, namely geese, swans, and cranes, but does not appear to be very helpful in understanding the movements of the more widely dispersing ducks or most other groups of birds. Young geese will tend to return to breed in the area in which they were hatched, even though competition might be less in goose populations breeding in another flyway. Mating in many ducks occurs on the winter range and even though a male had come south on one flyway, it will return with the female, perhaps on a different flyway. Consequently, vacant breeding areas are more rapidly repopulated by ducks than by geese.

Although Lincoln's analysis was confined to ducks and geese, some thought that it applied to other groups of birds as well. Everyone now realizes that the concept of four flyways, designated as the Atlantic, Mississippi, Central, and Pacific Flyways, was an oversimplification of an extremely complex situation involving crisscrossing of migration routes that vary from species to species. Flyways can be considered meaningful only in a very general way, even for waterfowl, and not generally applicable to other groups of birds. By determining relative abundances of dabbling ducks east of the Rocky Mountains, Frank Bellrose of the Illinois Natural History Survey presented a more realistic picture (Figure 13). Yet the four "Flyway" areas have been useful in regionalizing the harvest of waterfowl for areas of different vulnerability to hunting pressure. Bellrose also mapped the corridors for the diving ducks and showed heavy traffic similar to that of dabbling species through the Great Plains and relatively heavily used corridors from these central arteries eastward across the Great Lakes area to the Atlantic coast, terminating particularly in the vicinity of Chesapeake Bay. A fairly well-used corridor also extends along the Atlantic coast.

Figure 13.  Migration corridors used by dabbling ducks east of the Rocky Mountains during their fall migration (after Bellrose 1968).

The Harris' Sparrow provides an interesting example of a moderately narrow migration route in the interior of the country (Figure 14). This handsome sparrow is known to breed only in the narrow belt of stunted timber and brush along the northern limit of trees from the vicinity of Churchill, on the west shore of Hudson Bay, to the Mackenzie Delta 1,600 miles to the northwest. When this sparrow reaches the United States on its southward migration, it is most numerous in a belt about 500 miles wide between Montana and central Minnesota south through a relatively narrow path in the central part of the continent. Its winter range lies primarily from southeastern Nebraska and northwestern Missouri, across eastern Kansas and Oklahoma, and through a 150 miles-wide section of eastern Texas. The habitat preference of Harris' Sparrows for the coniferous forest-tundra transition on its breeding range also characterizes the structure of its habitat choice of shrubby patches within grasslands on its wintering range. Consequently it's narrow migratory pathway is west of the eastern deciduous forest, and even with deforestation the species has not widened its wintering area.

Figure 14.  Distribution and migration of Harris' Sparrow. This is an example of a narrow migration route through the interior of the country.

The Scarlet Tanager presents another extreme case of a narrowly converging migration route starting from its 1,900 mile-wide breeding range in the eastern deciduous forest between New Brunswick and Saskatchewan (Figure 15). As the birds move southward in the fall, their path of migration becomes more and more constricted until, at the time they leave the United States, all are included in the 600-mile belt from eastern Texas to the Florida peninsula. The boundaries continue to converge to less than 100 miles through Honduras and Costa Rica. The species winters in the heavily forested areas of northwestern South America including parts of Colombia, Ecuador, and Peru.

Figure 15.  Distribution and migration of the Scarlet Tanager. During the breeding season individual tanagers may be 1,500 miles apart in an east-and-west line across the breeding range. In migration, however, the lines gradually converge until in South America they are about 500 miles apart.

The Rose-breasted Grosbeak also leaves the United States through the 600-mile stretch from eastern Texas to Apalachicola Bay, but thereafter this grosbeak crosses the Gulf of Mexico and enters the northern part of its winter quarters in southern Mexico and these lines do not converge. However, the pathway of those individuals that continue to South America is considerably constricted by the narrowing of land through Central America (Figure 16). Although the cases cited represent extremes of convergence, a narrowing of migratory paths is the rule for the majority of North American birds. Both the shape of the continent and major habitat belts tend to constrict southward movement so that the width of the migration route in the latitude of the Gulf of Mexico is much less than in the breeding range. The American Redstart represents a case of a wide migration route, but even in the southern United States this path is still much narrower than the breeding range (Figure 17). These birds, however, cross all of the Gulf of Mexico and pass from Florida to Cuba and Haiti by way of the Bahamas, so that here their route is about 2,500 miles wide.

Figure 16.  Distribution and migration of the Rose-breasted Grosbeak. Though the width of the breeding range is about 2,500 miles, the migratory lines converge until the boundaries are only about 1,000 miles apart when the birds leave the United States.

Figure 17.  Distribution and migration of the Redstart. An example of a wide to migration route, birds of this species cross all parts of the Gulf of Mexico, or may travel from Florida to Cuba and through the Bahamas. Their route has an east-and-west width of more than 2,000 miles.

Figure 18.  Principal migration routes used by birds in passing from North America to winter quarters in the West Indies, Central America, and South America. Route 4 is the one used most extensively while only a few species make the 2,400 mile flight down Route 1 from Nova Scotia to South America.

The Arctic Tern follows the Atlantic Ocean route chiefly along the eastern side of the ocean in the eastern hemisphere. Likewise, vast numbers of seabirds such as auks, murres, guillemots, phalaropes, jaegers, petrels, and shearwaters follow this over-water route from breeding sites along coasts and on islands in the Northern and Southern Hemispheres.

Atlantic Coast Route and Tributaries

Route No. 3 (Figure 18) is a direct line of travel for Atlantic coast migrants enroute to South America, although it involves much longer flights. It is used almost entirely by landbirds. After taking off from the coast of Florida, there are only two intermediate land masses where migrants might pause for rest and food. Nevertheless, tens of thousands of birds of about 60 species cross the 150 miles from Florida to Cuba where many remain for the winter months. The others negotiate the 90 miles between Cuba and Jamaica but, from that point to the South American coast, there is a stretch of islandless ocean 500 miles across. The Bobolink so far outnumbers all other birds using this path that this route could be designated the "Bobolink route" (Figure 19). As traveling companions along this route, the Bobolink may meet vireos, kingbirds, and Common Nighthawks from Florida, Chuck-will's-widows from the Southeastern States, Black-billed and Yellow-billed cuckoos from New England, Gray-cheeked Thrushes from Quebec, Bank Swallows from Labrador, and Blackpoll Warblers from Alaska. Sometimes this scattered assemblage will be joined by a tanager or a Wood Thrush, but the "Bobolink route" is not used by the greatest number of migrants. Formerly, it was thought most North America landbirds migrated to Central America via the Florida coast, then crossed to Cuba, and finally made the short flight from the western tip of Cuba to Yucatan. A glance at the map would suggest this as a most natural route but, as a matter of fact, it is practically deserted except for a few swallows and shorebirds or an occasional landbird storm-driven from its normal course. What actually happens in the fall is that many of the birds breeding east of the Appalachian Mountains travel parallel to the seacoast in a more or less southwesterly direction and, maintaining this same general course from northwestern Florida, cross the Gulf of Mexico to the coastal regions of eastern Mexico. They thus join migrants from farther inland in using route No. 4 (Figure 18).

Routes used by Brant in eastern North American merit some detail because their flight paths were long misunderstood. These birds winter on the Atlantic coast, chiefly at Barnegat Bay, New Jersey, but depending upon the severity of the season and the food available, many winter south to North Carolina. Their breeding ground are in the Canadian arctic archipelago and on the coasts of Greenland. Careful studies have shown that the main body travels northward in spring along the coast to the Bay of Fundy, overland to Northumberland Strait, which separates Prince Edward Island from mainland New Brunswick and Nova Scotia. A minor route appears to lead northward from Long Island Sound by way of the Housatonic and Connecticut River valleys to the St. Lawrence River. After spending the entire month of May feeding and resting in the Gulf St. Lawrence, the eastern segment of the Brant population resumes its journey by departing overland from the Bay of Seven Island area, flying almost due north to Ungave Bay and from there to nesting grounds, probably on Baffin Island and Greenland. A smaller segment travels a route slightly north of west to the flocks take a more northwesterly course by descending the Fort George River to reach the eastern shore of James Bay. Upon their arrival at either of these two points on James Bay, the Brants of this western segment turn northward and proceed along eastern Hudson Bay to their breeding grounds in the Canadian Arctic.

The fall migration of Brant follows the routes utilized in the spring. During this season, the eastern population appears only on the western and southern shores of Ungave Bay before continuing their southward journey to the Gulf of St. Lawrence and beyond. Most of the birds of the western segment, instead of following the eastern shores of Hudson and James bays, turn southwestward across the former, by way of the Belcher Islands, to Cape Henrietta Maria, and from there south along the western shore of James Bay by way of Akimiski and Charlton Islands. At the southern end of James Bay, they are joined by those that have taken the more direct route along the east coasts of the bays and all then fly overland 570 miles to the estuary of the St. Lawrence River.

The Atlantic coast wintering area receives waterfowl from three or four interior migration paths, one of which is of primary importance, as it includes great flocks of Canvasbacks, Redheads, Greater and Lesser scaup, Canada Geese, and many American Black Ducks that winter in the waters and marshes of the coastal region south of Delaware Bay. The Canvasbacks, Redheads, and scaup coming from breeding grounds on the great northern plains of central Canada follow the general southeasterly trend of the Great Lakes, cross Pennsylvania over the mountains, and reach the Atlantic coast in the vicinity of Delaware and Chesapeake Bays. American Black Ducks, Mallards, and Blue-winged Teals that have gathered in southern Ontario during the fall leave these feeding grounds and proceed southwest. Many continue this route down the Ohio Valley, but others, upon reaching the vicinity of the St. Clair Flats between Michigan and Ontario, swing abruptly to the southeast and cross the mountains to reach the Atlantic coast south of New Jersey. This route, with its Mississippi Valley branch, has been fully documented by the recovery records of ducks banded at Lake Scugog, Ontario.

Figure 19.  Distribution and migration of the Bobolink. In crossing to South America, most of the Bobolinks use route 3 (Fig. 18), showing no hesitation in making the flight from Jamaica across an islandless stretch of ocean. It will be noted that colonies of these birds have established themselves in western areas, but in migration they adhere to the ancestral flyways and show no tendency to take the short cut across Arizona, New Mexico, and Texas.

Canvasbacks migrate from the prairie pothole region of the central United States and Canada to many wintering areas in the United States. This duck has been the subject of careful study, and its principle migration routes based on recovery of banded birds have been shown. These principle routes travel from the major breeding area in the prairie provinces of Canada and the northern prairies of the United States southeastward through the southern Great Lakes area to Chesapeake Bay, the chief wintering area (Figure 20). Relatively few Canvasbacks proceed southward along the Atlantic seaboard. A less important route extends off from the main trunk in the southern Minnesota region and goes south along the Mississippi Valley to points along the river. Other individuals of the prairie breeding population fly southward on a broad front to the gulf coast of Texas and the interior of Mexico, while some proceed southwestward on a relatively broad path to the northern Pacific coast.

Figure 20.  Principal migratory routes of the Canvasback. The major route of travel extends from breeding areas in central Canada southeast across the Great Lakes and either south down the Mississippi River or east to Chesapeake Bay (After Stewart et al. 1958).

When many of these species, including ducks, geese, American Robins, and Yellow-rumped Warblers, arrive at the Gulf coast, they spread out east and west for their winter sojourn. Others, despite the perils of a trip involving a flight of several hundred miles across the Gulf of Mexico, fly straight for Central and South America. This part of the route is a broad "boulevard" extending from northwestern Florida to eastern Texas and southward across the Gulf of Mexico to Yucatan and the Isthmus of Tehuantepec (Figure 18, route 4). This route appears to be preferred over the safer but more circuitous land or island routes by way of Texas or Florida. During the height of migration some of the islands off the coast of Louisiana are rewarding observation points for the student of birds, since the feathered travelers literally swarm over them.

Present detailed knowledge of the chief tributaries to the Mackenzie-Great Lakes-Mississippi Valley route relates primarily to waterfowl. Reference has been made already to the flight of American Black Ducks that reach the Mississippi Valley from southern Ontario. Some individuals of this species banded at Lake Scugog, Ontario, have been recaptured in succeeding seasons in Wisconsin and Manitoba, but the majority was retaken at points south of the junction of the Ohio River with the Mississippi indicating their main route of travel from southern Ontario.

A second route that joins the main artery on its eastern side is the one used by eastern populations of Snow Geese, including both blue and white phases, that breed mainly on Southampton Island and in the Fox Basin of Baffin Island. In the fall these geese work southward along the shores of Hudson Bay and, upon reaching the southern extremity of James Bay, take off on their flight to the great coastal marshes of Louisiana and Texas west of the Mississippi River delta.

Great Plains-Rocky Mountain Routes

Observations made in the vicinity of Corpus Christi, Texas have shown one of the short cuts (Figure 18, route 5) from the coastal bend of Texas to the shore of the Bay of Campeche that is part of the great artery of migration. Thousands of birds pass along the coast to the northern part of Veracruz, Mexico. Since coastal areas in Tamaulipas to the north are arid and unsuited for denizens of moist woodlands, it is probable that much, if not all, of this part of the route for these species is a short distance off shore. It is used by such woodland species as the Golden-winged, Worm-eating, and Kentucky warblers.

Pacific Coast Route

Figure 21.  The breeding range, wintering range, and main migration route of Ross' Goose. This is the only species of which practically all members breed in the Arctic, migrate south through the Canadian prairie, and upon reaching the United States, turn to the southwest rather than the southeast. The southern part of this route, however, is followed by some mallards, pintails, wigeons, and other ducks.

The southward route of long-distance migratory landbirds of the Pacific area extends chiefly through the interior of California to the mouth of the Colorado River and on to winter quarters in western Mexico (Figure 18, routes 6 and 7).

The movements of the Western Tanager show a migration route that is in some ways remarkable. The species breeds in the mountains from the northern part of Baja California and western Texas north to northern British Columbia and the southwestern headwaters of the Mackenzie River. Its winter range is in two discontinuous areas - southern Baja California and eastern and southwestern Mexico south to Guatemala (Figure 22). During spring migration the birds appear first in western Texas and the southern parts of New Mexico and Arizona about April 20 (Figure 23). By April 30 the vanguard has advanced evenly to an approximate east-west line across central New Mexico, Arizona, and southern California. By May 10 the easternmost birds have advanced only to southern Colorado, while those in the far west have reached northern Washington. Ten days later the northward advance of the species is shown as a great curve, extending northeastward from Vancouver Island to central Alberta and thence southeastward to northern Colorado. Since these tanagers do not reach northern Colorado until May 20, it is evident those present in Alberta on that date actually reached there by a route that carried them west of the Rockies to southern British Columbia and thence eastward across the still snowy northern Rocky Mountains.

Figure 22.  Breeding and wintering ranges of the Western Tanager. See Fig. 23 for the spring route taken by the birds breeding in the northern part of the range.

Figure 23.  Migration of the Western Tanager. The birds that arrive in eastern Alberta by May 20 do not travel northward along the eastern base of the Rocky Mountains, because the vanguard has then only reached northern Colorado. Instead the isochronal lines indicate that they migrate north through California, Oregon, and Washington and then cross the Rockies in British Columbia.