Migration of Birds

Migration of Birds

Routes of Migration

General Considerations
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
Through plotting accumulated banding data obtained in the 1930's, investigators became impressed by what appeared to be four broad, relatively exclusive flyway belts in North America. This concept, based upon analyses of the several thousand records of migratory waterfowl recoveries then available, led Fred Lincoln to conclude that, ". . . because of the great attachment of migratory birds for their ancestral flyways, it would be possible practically to exterminate the ducks of the West without seriously interfering with the supply of birds of the same species in the Atlantic and Mississippi flyways, and that the birds of these species using the eastern flyways would be slow to overflow and repopulate the devested areas of the West, even though environmental conditions might be so altered as to be entirely favorable." Since 1948, this model has served as the basis for administrative action by the Fish and Wildlife Service in setting annual migratory waterfowl hunting regulations.

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.

With our present knowledge of bird migration, recognizing distinct broad belts of migration down the North American continent encompassing groups of distinct populations or species is not realistic. About all we can say for sure now is that birds travel between certain breeding areas in the North and certain wintering areas in the South; that a few heavily traveled corridors are used by certain species; and that more generalized are routes followed by other species.

Narrow Routes
Some species exhibit extremely narrow routes of travel. The Red Knot and Purple Sandpiper, for example, are normally found only along the coasts because they are limited on one side by the broad waters of the ocean, and on the other by land and fresh water; neither of these habitat furnish conditions attractive to these species.

The Ipswich race of the Savannah Sparrow likewise has a very restricted migration range. It is known to breed only on tiny Sable Island, Nova Scotia, and it winters from that island south along the Atlantic coast to Georgia. It is rarely more than a quarter of a mile from the outer beach and is entirely at home among the sand dunes with sparse covering of coarse grass.

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.

Converging Routes
When birds start their southward migration, the movement necessarily involves the full width of the breeding range. Later, in the case of landbirds with extensive breeding ranges, there is a convergence of the lines of flight taken by individual birds owing, in part, to the conformation of the land mass and in part to the east-west restriction of habitats suitable to certain species. An example of this is provided by the Eastern Kingbird, which breeds in a summer range 2,800 miles wide from Newfoundland to British Columbia. On migration, however, the area traversed by the species becomes constricted until in the southern part of the United States the occupied area extends from Florida to the mouth of the Rio Grande, a distance of only 900 miles. Still farther south the migration path continues to converge and, at the latitude of Yucatan, it is not more than 400 miles wide. The great bulk of the species probably moves in a belt less than half this width.

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.

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