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, ex- 

 tending 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 narrow- 

 ing 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 



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