THE MIGRATION OF NORTH AMERICAN BIRDS 45 
blackpoll warblers from Alaska. Sometimes this scattered assem- 
blage will be joined by a tanager or a wood thrush but ‘the bobolink 
route” is not popular with the greater number of migrants, and al- 
though 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 termi- 
nus 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 moun- 
tains. 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 levelis 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, spar- 
rows, 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 any- 
where 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. Continu- 
ing southward the migrating flocks are constantly augmented by 
additions to their numbers as they pass over the great breeding 
erounds of central and southern Canada. Upon reaching the head- 
waters 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, appar- 
ently maintaining the same direction from northwestern Florida, 
-eross the Gulf to the coastal regions of eastern Mexico. On the other 
