part, to the conformation of the land mass and in part to the east-west 

 restriction of habitats suitable to certain species. An example of this 

 is provided by the eastern kingbird, which breeds in a summer 

 range 2,800 miles wide from Newfoundland to British Columbia. On 

 migration, however, the area traversed by the species becomes 

 constricted until in the southern part of the United States the 

 occupied area extends from Florida to the mouth of the Rio Grande, a 

 distance of only 900 miles. Still farther south the migration path 

 continues to converge, and, at the latitude of Yucatan, it is not more 

 than 400 miles wide. The great bulk of the species probably moves in 

 a belt less than half this width. 



The scarlet tanager presents another extreme case of a narrowly 

 converging migration route starting from its 1,900-mile-wide 

 breeding range in the eastern deciduous forest between New 

 Brunswick and Saskatchewan (Fig. 15). As the birds move 



Figure 15. Distribution and migration of the scarlet tanager. During the breeding sea- 

 son individual scarlet tanagers may be 1,500 miles apart in an east-and-west line 

 across the breeding range. In migration, however, the lines gradually converge until in 

 South America they are about 500 miles apart. 



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