Part 5

Figure 17.--Distribution and migration of the scarlet tanager. During the breeding season individual scarlet tanagers may be 1,900 miles apart in an east-and-west line across the breeding range. In migration, however, the lines converge until in southern Central America they are not more than 100 miles apart. For migration paths of other widths see figures 16, 18, and 19.]

The scarlet tanager presents another extreme case of narrowness of migration route (fig. 17), its breeding range extending in greatest width from New Brunswick to Saskatchewan, a distance of about 1,900 miles. As the birds move southward in 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. Continuing to converge through Honduras and Costa Rica, the boundaries there are not more than 100 miles apart. The species winters in northwestern South America, where it spreads out over most of Colombia, Ecuador, and Peru.

The rose-breasted grosbeak also leaves the United States through the 700-mile stretch from eastern Texas to Appalachicola Bay, but thereafter the lines do dot further converge, as this grosbeak enters the northern part of its winter quarters in Central America and South America through a door of about the same width (fig. 18).

While the cases cited represent extremes of convergence, a narrowing of the migratory path is the rule to a greater or less degree for the majority of North American birds. The shape of the continent tends to effect this, and so the width of the migration route in the latitude of the Gulf of Mexico is usually much less than in the breeding territory.

The redstart (_Setophaga ruticilla_) represents a notable case of a wide migration route, although even in the southern United States this is much narrower than the breeding range (fig. 19). These birds, however, cross all parts of the Gulf of Mexico and pass from Florida to Cuba and Haiti by way of the Bahamas, so that here their route has a width of about 2,500 miles.

In the following, the discussion of the principal routes of North American birds relates chiefly to the fall migration, for, except as otherwise noted, the spring flight generally retraces the same course. The routes indicated on the maps (figs. 20 and 21) must not be considered as representing paths with clearly defined borders, but rather as convenient subdivisions of the one great flyway that covers practically the entire width of the North American Continent and extends from the Arctic coast to South America.

ATLANTIC OCEANIC ROUTE

By reference to figure 20 it will be noted that route no. 1 is almost entirely oceanic, passing directly over the Atlantic Ocean from Labrador and Nova Scotia to the Lesser Antilles, and then through this group of small islands to the mainland of South America. It is not used by any of the smaller land birds, but is followed chiefly by thousands of water birds and by shore birds of several species, the adult golden plover being a notable example. Since it lies entirely over the sea, this route is definitely known only at its terminals and from occasional observations made on Bermuda and other islands in its course. Some of the shore birds that breed on the Arctic tundras of Mackenzie and in Alaska fly southeastward across Canada to the Atlantic coast and finally follow the oceanic route to the mainland of South America. The golden plover may accomplish the whole 2,400 miles without pause or rest, in fair weather the flocks passing Bermuda and sometimes even the islands of the Antilles without stopping. Although most birds make their migratory flights either by day or by night, the golden plover in this remarkable journey flies both day and night. As it swims lightly and easily it may make a few short stops along the way, and it has been actually seen resting on the ocean. Other shore birds have been observed busily feeding in that great area of ocean known as the Sargasso Sea, where thousands of square miles of floating seaweed teem with marine life.

The annual flight of the adult golden plover is so wonderful that it may be given in some detail, particularly since it is one of the exceptions to the general rule that spring and fall movements are over the same routes (fig. 22). After reaching the South American const the birds make a short stop and then continue overland to the pampas of Argentina, where they remain from September to March. Leaving their winter quarters, they cross northwestern South America and the Gidf of Mexico, reaching 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, reaching from the Arctic tundras to the pampas of Argentina. The older birds are probably accompanied by some of the young, perhaps those from early nestings, but most of the immatures leave their natal grounds late in summer and move southward through the interior of the country, re turning in spring over essentially the same course. The elliptical route is therefore used chiefly by fully adult birds.

ATLANTIC COAST ROUTE AND TRIBUTARIES

The Atlantic coast is a regular avenue of travel, and along it are many famous points for observing both land and water birds. About 50 different kinds of land birds that breed in New England follow the coast southward to Florida and travel thence by island and mainland to South America (fig. 20, route 2). As will be seen from the map, a seemingly natural and convenient highway extends through the Bahamas, Cuba, Hispaniola, Puerto Kico, and the Lesser Antilles to the South American coast. Resting places are afforded at convenient intervals, and at no time need the aerial travelers be out of sight of land. It is not, however, the favored highway, and only about 25 species of birds go beyond Cuba to Puerto Rico along this route to their winter quarters, while only 6 species are known to reach South America by way of the Lesser Antilles. The obvious drawback is lack of adequate food. The total area of all the West Indies east of Puerto Rico is less than that of Rhode Island, so that if only a small part of the birds of the eastern United States were to travel this way, it is doubtful whether even the luxuriant flora and fauna of tropical habitats would provide food sufficient for their needs.

In the northern part of the Atlantic coast route is a tributary route used by the brant (_Branta bernicla_) that is of special interest. The southward movement of these birds is chiefly along the western shores of Hudson Bay and thence southeastward to the Atlantic coast. Returning in spring, they follow the coast line north to the Gulf of St. Lawrence and then fly almost due north to their breeding grounds on the west coast of Greenland and the islands of the Arctic archipelago. The round trip is therefore in the form of a great ellipse, probably 3,000 miles long by 1,000 miles wide.

The Atlantic flyway receives accretions of waterfowl from three or four interior migration paths, one of which is of first importance, as it includes great flocks of canvasbacks (_Nyroca valisineria_), redheads (_N. americana_), scaup ducks (_N. marila_ and _N. affinis_), Canada geese, and many of the black ducks that winter in the waters and marshes of the coastal region south of Delaware Bay. The canvasbacks, redheads, and scaups come from their breeding grounds on the great northern plains of central Canada, follow the general southeasterly trend of the Great Lakes (fig. 21), cross Pennsylvania over the mountains, and reach the Atlantic coast in the vicinity of Delaware and Chesapeake Bays. Black ducks, mallards, and blue-winged teals (_Querquedula discors_) that have gathered in southern Ontario during the fall leave these feeding grounds and proceed southwest over a course that is apparently headed for the Mississippi Valley. Many do continue this route down the Ohio Valley, but others, upon reaching the vicinity of Lake St. Clair, between Michigan and Ontario, swing abruptly southeast and, crossing the mountains in a single flight, reach the Atlantic coast south of New Jersey. This route, with its Mississippi Valley branch, has been fully demonstrated by the recovery records of ducks banded by a cooperator of the Biological Survey at Lake Scugog, Ontario.

The white-winged scoter (_Melanitta deglandi_), which also breeds in the interior country from northern North Dakota north to the Arctic coast, is another bird having an elliptical migration route, so far as those wintering on the Atlantic coast are concerned. This duck breeds only near fresh water and winters entirely on the ocean along both the Atlantic and Pacific coasts of the United States. Those wintering on the Atlantic side leave their breeding grounds west of Hudson Bay and fly 1,500 miles almost due east to the most eastern part of Labrador, whence they proceed southward across the Gulf of St. Lawrence to their winter home, which extends from southwestern Maine to Chesapeake Bay. The spring flight is made by an interior route that traverses the valleys of the Connecticut, Hudson, and Ottawa Rivers, and thence passes west and north to the breeding grounds.

A study of the Canada geese that winter abundantly in the waters of Back Bay, Va., and Currituck Sound, N. C., reveals another important tributary to the Atlantic coast route. Banding has shown that the principal breeding grounds of these birds are among the islands and on the eastern shores of Hudson Bay (fig. 21). From this region they move south in fall to the point of lower Ontario between Lakes Erie and Huron. Some of the banded geese are recovered in the Mississippi Valley, but the great majority are retaken either on their breeding grounds or on the Atlantic coast south of Delaware Bay, showing another instance of a long cross-country flight by waterfowl. Although Canada geese are abundant in migration on the coast of New England, the birds taken there do not include any that were banded in southern Ontario. Again, banding has shown that the New England visitants come from other breeding areas, chiefly Newfoundland and the desolate coast of Labrador, and that their migration is entirely coastwise.

Still another cross-country route between the Mississippi Valley and the Atlantic coast may be briefly described. While not yet well understood, a hitherto unsuspected migration route across the Alleghenies to the Mississippi Valley has been revealed by the banding of blue-winged teal, on the coastal sawgrass marshes of South Carolina. Birds marked in these marshes have been retaken in Tennessee and Kentucky as well as in States farther north in the Mississippi Valley. Several species of shoal-water ducks, including this dainty little teal and the shoveler (_Spatula clypeata_), are more or less common winter residents in the South Carolina marshes, but are less common or even decidedly rare in most of the coastal marshes farther north, so this cross-country route connecting two main arteries of migration seems to be of considerable importance.

Referring again to figure 20, it is noted that route no. 3 presents a much more direct line of flight for the Atlantic coast migrants to South America than the others, although it involves much longer flights. It is used almost entirely by land birds. After taking off from the coast of Florida the migrants find only two land masses on the way where they can pause for rest and food. Nevertheless, tens of thousands of birds of some 60 species cross the 150 miles from Florida to Cuba, where about half of them elect to remain for the winter. The others fly the 90 miles between Cuba and Jamaica. From that point to the South American coast, however, there is a stretch of unbroken ocean fully 500 miles across, and scarcely a third of the North American migrants leave the forested mountains of Jamaica to risk the perils of this ocean trip. Chief among those that do is the bobolink (_Dolichonyx oryzivorus_), which so far outnumbers all other birds using this fly way that route no. 3 may well be called "the bobolink route" (fig. 23). As traveling companions along this route the bobolinks may meet vireos, kingbirds, and nighthawks from Florida; the chuck-will's-widow (_Antrostomus carolinensis_) of the Southeastern States; black-billed and yellow-billed cuckoos (_Coccyzus erythropthalmus_ and _C. americanus_) from New England; gray-cheeked thrushes from Quebec, bank swallows (_Riparia riparia_) 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 popular with the greater number of migrants, and although many individuals traverse it, they are only a small fraction of the multitudes of North American birds that spend the winter in South America.

MACKENZIE VALLEY-GREAT LAKES-MISSISSIPPI VALLEY ROUTE AND TRIBUTARIES

Easily the longest flyway of any in the Western Hemisphere is that extending from the Mackenzie Valley past the Great Lakes and down the Mississippi River, including its tributaries. Its northern terminus is on the Arctic coast in the regions of Kotzebue Sound, Alaska, and the mouth of the Mackenzie River, while its southern end lies in Patagonia (fig. 21). During the spring migration some of the shore birds traverse the full extent of this great path, and it seems likely that the nighthawk, the barn swallow, the blackpoll warbler, and individuals of several other species that breed north to Yukon and Alaska must twice each year cover the larger part of it.

For more than 3,000 miles--from the mouth of the Mackenzie to the Delta of the Mississippi--this flyway is uninterrupted by mountains. In fact, there is not even a ridge of hills on the route high enough to interfere with the movements of the feathered travelers, and the greatest elevation above sea level is less than 2,000 feet. Well timbered and watered, the entire region affords ideal conditions for the support of its great hosts of migrating birds. This route is followed by such vast numbers of ducks, geese, shore birds, blackbirds, sparrows, warblers, and thrushes, that observers stationed at favorable points in the Mississippi Valley during the height of migration can see a greater number of species and individuals than can be noted anywhere else in the world.

Starting in the region of Kotzebue Sound, Alaska, the route extends eastward across northern Alaska and joins another that has its origin at the mouth of the Mackenzie River (fig. 21). The line of flight then trends a little east of south through the great lake system of central Canada, where it is joined by 2 or 3 other routes from the northeast that have their origin on the central Arctic coast. Continuing southward the migrating flocks are constantly augmented by additions to their numbers as they pass over the great breeding grounds of central and southern Canada. Upon reaching the headwaters of the Missouri and Mississippi Rivers the route follows these streams to the Gulf coast. Arriving in this latitude many species, including ducks and geese, the robin, the myrtle warbler, and some others 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, strike out boldly for Central America and South America. This part of the route is a broad "boulevard" extending from northwestern Florida to eastern Texas and reaching southward across the Gulf of Mexico to Yucatan and the Isthmus of Tehuantepec (fig. 20, route 4).

Many of the birds that breed east of the Allegheny Mountains parallel the sea coast as they move southwestward in fall and, apparently maintaining the same direction from northwestern Florida, cross the Gulf to the coastal regions of eastern Mexico. On the other hand, the birds that have come south directly through the Mississippi Valley and the region west to the Rocky Mountains reach the coastal plains of Mississippi, Louisiana, and Texas and continue directly across the Gulf. The great majority of North American birds seeking winter homes in the Tropics thus elect the short cut across the Gulf of Mexico in preference to the longer although safer land or island journey by way of Texas or Florida. During the height of the migration some of the islands off the coasts of Louisiana and Texas are wonderful observation places for the student of birds, as the feathered travelers literally swarm over them.

One of the short cuts (fig. 20, route 5) that may be considered a part of this great water artery of migration extends a few hundred miles from the coast of Texas to the northern part of the State of Vera Cruz. As the neighboring coast is arid and thus entirely unsuited to the needs of birds that are frequenters of moist woodlands, it is not surprising to find that this Gulf route is used by such woodland species as the golden-winged warbler (_Vermivora chrysoptera_), the worm-eating warbler (_Helmitheros vermivorus_), and the Kentucky warbler (_Oporornis formosus_).

Formerly it was thought that most of the North American birds that migrate to Central America made a leisurely trip along the Florida coast, crossed to Cuba, and thence 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 shore birds, or an occasional land bird storm-driven from its accustomed course.

Present knowledge of the chief tributaries to the Mackenzie Valley-Great Lakes-Mississippi Valley highway relates chiefly to waterfowl. Reference already has been made to the flight of the black ducks (p. 42) 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 have been retaken at points south of the junction of the Ohio River with the Mississippi, definitely indicating their route of travel from southern Ontario.

A second route that joins the main artery on its eastern side is the one used by the blue goose (fig. 10), the migration route of which is probably more nearly due north and south than that of any other North American bird. The breeding grounds, which only recently have been discovered, are in the Fox Basin region of Baffin Island and on Southampton Island. In fall these geese work southward, chiefly along the eastern shore of Hudson Bay, and upon reaching the southern extremity of James Bay they take off for what is practically a nonstop flight to the great coastal marshes of Louisiana west of the delta of the Mississippi River (fig. 21). In some seasons the flocks make intermediate stops among the islands and sand bars of the Mississippi, as they are occasionally common in the general vicinity of Memphis, Tenn. Most of the birds push on, however, and during the period from the first of November to the last of March fully 90 percent of the species are concentrated in the area between the Sabine and the Mississippi Rivers. On the return trip northward there is sometimes a tendency for some of the blue geese to veer off toward the Northwest, as they are occasionally abundant in eastern South Dakota and southeastern Manitoba. It is of particular interest to note that while some other geese and many ducks start their northward journey at the first sign of awakening spring, the blue goose remains in its winter quarters until the season there is far advanced, seemingly aware that its own breeding grounds in the Arctic are still in the grip of winter.

As shown by the route map (fig. 21) a great western highway also has its origin in the Mackenzie River delta area and in Alaska. This is used chiefly by the pintail and the baldpate (Mareca americana), which fly southward through eastern Alberta to western Montana. Some localities in this area, as for example, the National Bison Range, at Moiese, Mont., normally furnish food in such abundance as to induce these birds to pause in their migratory movement. Upon resuming travel, some flocks move almost directly west across Idaho to the valley of the Columbia River, from which they turn abruptly south to the interior valleys of California. Others leave the Montana feeding and resting areas and turn southeastward across Wyoming and Nebraska to join the flocks that either are moving southward through the Great Plains, or are to continue across Arkansas to the main Mississippi Valley fly way.

Many redheads that breed in the Bear River marshes in Utah, take a westerly route across Nevada to California, but some leave these breeding grounds and fly northeastward across North Dakota and Minnesota to join the flocks of these ducks that come out of the prairie regions of Canada, and travel southeastward to the Atlantic coast. This route can be well traced by the records of ducks banded in summer in the Bear River marshes and retaken the following fall at points in eastern Montana, Wyoming, South Dakota, North Dakota, Minnesota, Wisconsin, Michigan, and Maryland.