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.