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). [Illustration: B4501M Figure 18.--Distribution and migration of the rose-breasted grosbeak. Though the width of the breeding range is about 2,500 miles, the migratory lines converge until the boundaries are only about 700 miles apart when the birds leave the United States. For migration paths of other widths see figures 16, 17, and 19.] 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. [Illustration: B4500M Figure 19.--Distribution and migration of the redstart. An example of a wide migration route, since birds of this species cross all parts of the Gulf of Mexico, or may travel from Florida to Cuba and through the Bahamas. Their fly way thus has an east and west width of more than 2,000 miles. For migration paths of greater or less extent see figures 16, 17, and 18.] 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. [Illustration: B1032M