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.