Routes of Migration
While it is beyond question that certain general directions of flight are constantly followed by migratory birds, it is well to remember that the term "migration route" is to some extent a theoretical concept concerned entirely with the lines of general advance or retreat of a species, rather than the exact course followed by individual birds. Even the records of banded birds usually show no more than the place of banding and recovery, and one must have recourse to intermediate records and to reasoning based on probabilities to fill in details of the route actually traversed between the two points.
There is also infinite variety in the routes covered during migration by different species. In fact, the choice of migration highways is so wide that it seems as if the routes of no two species coincide. Differences in distance traveled, time of starting, speed of flight, geographical position, latitudes of breeding and of wintering grounds, and in other factors, all contribute to this great variation of migration routes. Nevertheless, there are certain factors that serve to guide the avian travelers along more or less definite lines, and it is possible to define general lines of migration for the majority of species.
It has been frequently observed that migrating birds have a tendency to follow major topographic lines on the earth's surface when their trend is in the general direction of the birds' journey. Bird migration is generally thought of as a north-and-south movement, with the lanes of heavier concentration following the coasts, mountain ranges, and principal river valleys. To a considerable extent this is the case, particularly in North America, where the coast lines, mountain chains, and larger rivers in general run north and south. In cases where the migration is a long one, however, the notion must be abandoned that the birds' flight is always restricted to narrow routes that follow river valleys and the like, as many species seem to disregard utterly such apparently good natural highways. For example, the Arkansas River has a general east-and-west course for a great part of its length, and while it does constitute a route for many perching birds en route from the Mississippi Valley to the Rocky Mountain region, some of the hawks and many ducks and shore birds pay the valley scant attention. They may arrest the autumn journey to feed among the cottonwoods or along sand bars, but when ready to resume their flight they leave the river and fly directly south over the more or less arid region that lies between the Arkansas and the Rio Grande.
Wide and narrow migration lanes
When birds start their southward migration the movement necessarily involves the full width of the breeding range. Later there is a convergence of the lines of flight taken by individual birds, owing to the conformation of the land mass, and as the species proceeds southward the width of the occupied region becomes less and less. An example of this is provided by the common kingbird, which breeds from Newfoundland to British Columbia, a summer range 2,800 miles wide. On migration, however, its paths converge 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, and still farther south the migration path is further restricted. In the latitude of Yucatan it is not more than 400 miles wide, and it is probable that the great bulk of the species moves in a belt that is less than half that width.
A migration route, therefore, may be anything from a narrow path that adheres closely to some definite geographical feature, such as a river valley or a coast line, to a broad boulevard that leads in the desired direction and which follows only the general trend of the land mass. Also it is to be remembered that whatever main routes are described, there remains a multitude of tributary and separate minor routes. In fact, with the entire continent of North America crossed by migratory birds, the different groups or species frequently follow lines that may repeatedly intersect those taken by others of their own kind or by other species. The arterial routes, therefore, must be considered merely as indicating paths of migration on which the tendency to concentrate is particularly noticeable.
In considering the width of migration lanes it will be obvious that certain species, as the knot and the purple sandpiper, which are normally found only along the coasts, must have extremely narrow routes of travel. They are limited on one side by the broad waters of the ocean, and on the other by land and fresh water, both of which are unsuited to furnish the food that is desired and necessary to the well-being of these species.
Among land birds that have a definite migration, the Ipswich sparrow has what is probably the most restricted migration range of any species. It is known to breed only on Sable Island, Nova Scotia, and it winters along the Atlantic coast south to Georgia. Living constantly within sound of the surf, it is rarely more than a quarter of a mile from the outer beach, and is entirely at home among the sand dunes and their sparse covering of coarse grass.
Harris's sparrow supplies an interesting example of a narrow migration route in the interior of the country (fig. 9.) This fine, large finch is known to breed only in the region from Churchill, on the west shore of Hudson Bay, northwest to the shores of Great Bear Lake. Very few actual breeding records of the species are available, but these are sufficient to indicate that the breeding range is in the strip of country characterized by more or less stunted timber just south of the limit of trees. When it begins its fall migration, this species necessarily covers the full width of its breeding area. Then it proceeds almost directly south, or slightly southeasterly, the area covered by the majority of the birds becoming gradually constricted, so that by the time it reaches the United States it is most numerous in a belt about 500 miles wide, extending across North Dakota to central Minnesota. Harris's sparrows are noted on migration with fair regularity east to the western shore of Lake Michigan, and west to the foothills of the Rocky Mountains, but the great bulk of the species moves north and south through a relatively narrow path in the central part of the continent. Present knowledge suggests that the reason for this narrow migration range is the close association that Harris's sparrow maintains with a certain type of habitat' including brushy places, thickets, edges of groves, and weed patches. While these environmental conditions are found in other parts of the country, the region crossed by this sparrow presents almost a continuous succession of habitat of this type. Its winter range extends from southeastern Nebraska and northwestern Missouri, across eastern Kansas and Oklahoma and through a narrow section of central Texas, at places hardly more than 150 miles wide.
The scarlet tanager presents another extreme case of narrowness of migration route (fig. 10), 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 600-mile stretch from eastern Texas to Apalachicola Bay, but thereafter the lines do not 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. 11).
Although 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 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. 12). 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.
The flyways
In 1935, as a result of studies of banding data, the author discovered the existence of the four great flyway systems. This discovery, based upon analyses of the several thousand records of the recovery of migratory waterfowl then available, was announced by the Biological Survey (Lincoln, 1935c) and, beginning in 1948, it has served as the basis for administrative action by the Fish and Wildlife Service in the annual hunting regulations.
Although this study was confined to this one family of birds there is a growing mass of evidence in support of the belief that all populations of migratory birds adhere with more or less fidelity to their respective flyways. The terms "flyway" and "migration route" have in the past been used more or less as synonyms but the modern concept of a flyway is that it is a vast geographic region with extensive breeding grounds and wintering grounds connected with each other by a more or less complicated system of migration routes. Each flyway has its own populations of birds, even of those species that may have a continental distribution. The breeding grounds of one or more flyways may (and usually do) overlap broadly, so that during the nesting season extensive areas may be occupied by birds of the same species but which belong to different flyways.
The maps (figs. 13, 14, 15, and 16) show the flyways as they are now understood. It should be pointed out, however, that in the other maps used in this bulletin, the entire range of a species is shown without any attempt to distinguish by flyways the different populations. As banding data accumulate for the nongame species, this distinction will ultimately be possible, but for the time being, consideration of their migrations must be chiefly by routes.
The following 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 map (fig. 17) must not be considered as representing paths with clearly defined borders, but rather as convenient subdivisions of the four great flyways that, as indicated above, cover practically the entire width of the North American Continent and extend from the Arctic coast to South America.
Atlantic oceanic route
By reference to figure 17 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. This is not a popular route and its chief claim to fame is that it is the fall route used by most of the adult eastern golden plovers, and probably by some other shore birds. 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 tundra of Mackenzie and Alaska fly southeastward across Canada to the Atlantic coast and finally follow this 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 seen actually resting on the ocean. Other shore birds have been observed busily feeding in the 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 eastern 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. 18). After reaching the South American coast 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 Gulf 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 tundra 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 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 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. 17, route 2). As will be seen from the map, a seemingly natural and convenient highway extends through the Bahamas, Cuba, Hispaniola, Puerto Rico, 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 draw-back 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. Nevertheless, many thousands of coots, widgeons, pintails, blue-winged teal, and other waterfowl and shorebirds regularly spend the winter season in the coastal marshes and the inland lakes and ponds of Cuba, Hispaniola, and Puerto Rico.
The map (fig. 17) also will show that route No. 3 presents a direct line of travel for Atlantic coast migrants en route to South America, although it involves much longer flights. It is used almost entirely by land birds. After taking off from the coast of Florida there are only two intermediate land masses where the migrants may 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 this number elect to remain for the winter months. The others do not hesitate to fly the 90 miles between Cuba and Jamaica, but from that point to the South American coast there is a stretch of islandless ocean fully 500 miles across. Scarcely a third of the North American migrants leave the forested mountains of Jamaica to risk the perils of this ocean trip. Chief among these is the bobolink, which so far outnumbers all other birds using this route that it may be well called the "bobolink route" (fig. 19). As traveling companions along this route, the bobolink may meet a vireo, a kingbird, and a nighthawk from Florida; the chuck-wills-widow of the Southeastern States; the black-billed and the yellow-billed cuckoos from New England; the gray-cheeked thrush from Quebec; bank swallows from Labrador; and the blackpolled warbler 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.
Formerly it was thought that most of the North American land 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 shorebirds, or an occasional land bird storm-driven from its accustomed course. What actually happens is that in the fall many of the birds that breed east of the Allegheny Mountains travel parallel to the seacoast in a more or less southwesterly direction and, apparently maintaining this same general course from northwestern Florida, cross the Gulf of Mexico to the coastal regions of eastern Mexico.
The routes used by the Atlantic brant merit some detail for the reason that these were long misunderstood. These birds winter on the Atlantic coast, chiefly at Barnegat Bay, N. J., but (depending upon the severity of the season and the food available) south also to North Carolina. Their breeding grounds are in the Canadian arctic archipelago and on the coasts of Greenland. According to the careful studies of Lewis (1937), the main body travels northward in spring along the coast to the Bay of Fundy, hence overland to Northumberland Strait, which separates Prince Edward Island from the mainland of New Brunswick and Nova Scotia. A minor route appears to lead northward from Long Island Sound by way of such valleys as those of the Housatonic and Connecticut Rivers, and on across southern Quebec to the St. Lawrence River.
After spending the entire month of May feeding and resting in various parts of the Gulf of St. Lawrence, the eastern segment of the brant population suddenly resumes its journey by crossing to the north shore of the St. Lawrence estuary. The Bay of Seven Islands, in this general region, is the point of departure for long overland flights that are made by the two segments of the population. The eastern and larger of these appears to fly almost due north to Ungava Bay and from there to nesting grounds, probably in Baffin Island and Greenland. The smaller segment travels a route that is but slightly north of west to the southeastern shores of James Bay, although somewhere to the east of that area some of the flocks take a more northwesterly course, descending the Fort George River to reach the eastern shore of James Bay about two-thirds of the distance north of its southern extremity. Upon their arrival at either of these two points on James Bay, the brant of this western segment turn northward and proceed through the eastern part of Hudson Bay to their breeding grounds, probably in western Baffin Island, Southampton Island, and other islands in the Canadian Arctic.
In general, the fall migration of the brant follows the routes utilized in the spring. At this season, when gathering for the flight of 570 miles to the St. Lawrence River, they appear only on the western and southern shores of Ungava Bay. Also, it appears that 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 shores 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 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, redheads, scaup ducks, 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, 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 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 the St. Clair Flats, between Michigan and Ontario, swing abruptly to the 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 at Lake Scugog, Ontario.
The white-winged scoter, which also breeds in the interior of the continent from northern North Dakota north to the Arctic coast, was at one time credited with an elliptical migration route, at least insofar as those wintering on the Atlantic coast are concerned. This sea duck nests only near fresh water but spends the winters on the ocean along both the Atlantic and Pacific coasts of the United States. It migrates over land surfaces mainly at night and it is now believed that after working northward to the waters of Long Island Sound it starts up the valleys of the Hudson and Connecticut Rivers and flies overland to the Great Lakes, from which region it continues west and northwest to the breeding grounds, returning to its winter quarters over the same route. Early ideas alleging an elliptical route probably arose from the fact that great numbers of first-year nonbreeding birds regularly pass up the New England coast, cross the Gulf of St. Lawrence and spend the summer loafing off the coast of Labrador. In the fall these birds form into large flocks and retrace their flight, chiefly during daylight hours, to winter quarters from southeastern Maine south at least to Chesapeake Bay. As it was not known that the white-winged scoter does not usually breed until it is 2 years old, and since the southward movement of yearling birds was conspicuous while the travels of those from the nesting grounds were chiefly at night, the theory was advanced that the latter flew 1,500 miles due east from the region west of Hudson Bay to the coast of Labrador, thence southward to the known winter quarters.
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 shore of Hudson Bay. From this region they move south in autumn to the point of Lower Ontario between Lakes Erie and Huron. Occasionally one of these geese will be 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 the Canada goose is abundant in migration on the coast of New England, the birds taken there rarely include any of those banded in southern Ontario. The northeastern population of these geese comes from breeding areas in New England, the Maritime Provinces of Canada, Newfoundland, and the desolate coast of Labrador, their migration being entirely coastwise. Still another cross-country route between the Mississippi Valley and the Atlantic coast remains to be briefly described. It is not yet well understood, but the banding of ducks such as the blue-winged teal on the coastal saw-grass marshes of South Carolina has revealed that there is a migration route across the Appalachians to the Mississippi Valley. Birds marked in these marshes have been retaken in Tennessee and Kentucky, as well as in other States farther north in the Mississippi Flyway.
Mackenzie Valley—Great Lakes—Mississippi Valley route and tributaries
Easily the longest route 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. 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 blackpolled 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 route is uninterrupted by mountains. In fact, there is not even a ridge of hills 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. It 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 daring 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. The line of flight then trends a little east of south through the great lake system of central Canada, where it is joined by two or three 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 the 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. 17, route 4). In other words, when most of the birds (chiefly shore birds and land birds) that have come south directly through the Mississippi Valley have reached the coastal plains of Mississippi and Louisiana, they continue directly across the Gulf of Mexico. This route appears to have preference over the safer but more circuitous land or island routes by way of Texas and Florida. During the height of migration some of the islands off the coast of Louisiana are wonderful observation points for the student of birds, as the feathered travelers literally swarm over them.
Present detailed knowledge of the chief tributaries to the Mackenzie—Great Lakes—Mississippi Valley highway relates chiefly to waterfowl. Reference already has been made to the flight of the black ducks (p. 58) 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, 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 in recent years have been discovered, are mainly 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. 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.
Great Plains—Rocky Mountain routes
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 American widgeon or baldpate, 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 Colorado to join the flocks that are moving southward through the Great Plains (fig. 15).
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. A few of them even travel southeastward to the Atlantic coast. This route can be 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, Ohio, and Maryland. Great numbers, however, follow another route from these marshes across the mountains in an easterly direction, where it almost immediately turns southward through Colorado and New Mexico, and continues to winter quarters in the Laguna Madre off the coast of Texas or in the Valley of Mexico (fig. 17, route 6). This route also represents the travels of many of the land birds of the Rocky Mountain region. Such birds perform comparatively short migrations, most of them being content to stop when they reach the middle districts of Mexico, only a few passing beyond the southern part of that country.
Observations made in the vicinity of Corpus Christi, Tex., have shown one of the short cuts (fig. 17, route 5) that is, in effect, a part of the great artery of migration. Thousands of birds pass along the coast to the northern part of the State of Vera Cruz. As the coast of the State of Tamaulipas to the north is arid and so entirely unsuited to the needs of birds that are frequenters of moist woodlands, it is probable that much or all of this part of the route is a short distance off shore. It is used by such woodland species as the golden-winged warbler, the worm-eating warbler, and the Kentucky warbler.
Pacific coast route
Although it does present features of unusual interest, the Pacific coast route is not of so great importance as some of the others described. Because of the equable conditions that prevail, many species of birds along the coast from the Northwestern States to southeastern Alaska either do not migrate at all or else make relatively short journeys. This route has its origin chiefly in Alaska, the general region of the delta of the Yukon River marking its northern terminus, although a few species join it after a flight westward along the Alaskan Arctic coast. Some of the scoters and other sea ducks of the north Pacific region, and the diminutive cackling goose which breeds in the delta of the Yukon River, use the coastal sea route for all or most of their southward flight. The journey of the cackling goose, as shown by return records from birds banded at Hooper Bay, Alaska, has been traced southward across the Alaska Peninsula and apparently across the Gulf of Alaska to the Queen Charlotte Islands, the birds following the coast line south to near the mouth of the Columbia River. There the route swings toward the interior for a short distance before continuing south by way of the Willamette River Valley. The winter quarters of the cackling goose are chiefly in the vicinity of Tule Lake, on the Oregon-California line, and in the Sacramento Valley of California, though a few push on to the San Joaquin Valley.
A tributary of this flyway is followed by Ross's goose which is now known to breed only in the Perry River district south of the Queen Maud Gulf on the central Arctic coast of Canada (fig. 20). Its fall migration appears to be southwest and south across the barren grounds to Great Slave and Athabaska Lakes, where it joins thousands of other waterfowl bound for their winter homes along the eastern coast of the United States and the Gulf of Mexico. But when the Ross's geese have traveled south approximately to the northern boundary of Montana, they separate from their companions, and turning to the southwest cross the Rocky Mountains and settle for the winter in California.
The route taken by the white-winged scoters that winter on the Atlantic coast already has been indicated (p. 59). Some birds of this species, however, winter on the Pacific coast from Puget Sound south to southern California. Their passage by thousands up and down the coast has been noted as far north as northwestern British Columbia. The species is known to nest in Alaska, which may be the home of some, at least, of the scoters that winter on the Pacific coast. If such be the case, however, it must be admitted that a part of the route taken by the birds when on migration is unknown, though very few observations are available from the interior of northern British Columbia, across which the route may lie.
The southward route of those migratory land birds of the Pacific coast that in winter leave the United States extends chiefly through the interior of California to the mouth of the Colorado River and on to winter quarters in western Mexico.
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 northeastern British Columbia and southwestern Mackenzie. Its winter range is in two discontinuous areas—southern Baja California and eastern Mexico south to Guatemala (fig. 21). On the spring migration the birds enter the United States about April 20, appearing first in western Texas and the southern parts of New Mexico and Arizona (fig. 22). By April 30 the vanguard has advanced evenly to an approximate east-and-west line across central New Mexico, Arizona, and southern California. But 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 Albert and thence southeastward to northern Colorado. Since these tanagers do not reach northern Colorado until May 20, it is evident that those present in Alberta on that date, instead of traveling northward through the Rocky Mountains, which from the location of their summer and winter homes would seem to be the natural route, reached there by the Pacific coast route to southern British Columbia and thence across the mountains, despite the fact that these are still partly covered with snow at that time.
Pacific oceanic route
The route of the Pacific golden plover is fully as interesting and as remarkable as the elliptical course followed by its eastern cousin (fig. 18). The breeding range of the eastern golden plover extends through Arctic America west to the northern coast of Alaska where, in the vicinity of Point Barrow, it meets the nesting grounds of the Pacific form, which is really an Asiatic subspecies. It breeds chiefly in the Arctic coast region of Siberia and merely overflows onto the Alaskan coast, some of the birds probably migrating south along the coast of Asia to winter quarters in Japan, China, India, Australia, New Zealand, and Oceania, including the Hawaiian Islands, the Marquesas Islands, and the Low Archipelago. Golden plovers in migration have been observed at sea on a line that apparently extends from these islands to the Aleutians, and it therefore appears certain that at least some of the Alaskan birds make a nonstop flight across a landless sea from Alaska to Hawaii. While it would seem incredible that any birds could lay a course so straight as to attain these small oceanic islands, 2,000 miles south of the Aleutians, 2,000 miles west of Baja California, and nearly 4,000 miles east of Japan, the evidence admits only the conclusion that year after year this transoceanic round-trip journey between Alaska and Hawaii is made by considerable numbers of golden plovers.