Many smiliar cases might be mentioned, such as the black- 

 throated blue warblers still observed in the mountains of Haiti during 

 the middle of May when others of this species are en route through North 

 Carolina to New England breeding grounds. Redstarts and yellow 

 warblers, evidently the more southern breeders, are seen returning 

 southward on the northern coast of South America just about the 

 time the earliest of those breeding in the North reach Florida on their 

 way to winter quarters. Examples of the Alaska race of the yellow 



Figure 2. Isochronal migration lines of the black-and-white warbler, showing a very 

 slow and uniform migration. The solid lines connect places at which these birds arrive 

 at the same time. These birds apparently advance only about 20 miles per day in 

 crossing the United States. 



19 



