38 



BULLETIN 185, U. S. DEPARTMENT OF AGPJCULTUBE. 



All example or two will make this clear. The black-and-white 

 warbler breeds from South Carolina to New Bininswick (see fig. 15). 

 In the southerji part of its range it nests in April. New Brunswick, 

 however, is scarcely reached by the earliest birds before the middle 

 of May (see fig. 16), as the species occupies about 50 days in crossing 



Fig. 17. — Isochronal migration lines of the gi-ay-cheeked thrush (Hylockhla alidx). An example of rapid 

 migration. The whole 4,000 miles from Louisiana to Alaska is passed over in about 30 days, or about 

 130 miles per day. The last part of the route the speed is several times what it is in the central Missis- 

 sippi Valley. (See p. 27.) For an example of the opposite, a slow migration, see the black-and-white 

 warbler (fig. 16). 



the breeding range. It is probable that 60 days is about the shortest 

 l^ossible time in which such a bird could build a nest, rear its j^oung, 

 molt, and be ready for the retimi journey; and if so, then no New 

 Brunswick black-and-white warbler is ready to start south before the 

 middle of July, and 50 days for the trip would bring the earliest 

 migrants to the Gulf States in September. Yet both old birds and 



