10 CIRCULAR 363, U. 8S. DEPARTMENT OF AGRICULTURE 
Florida, the West Indies, central Mexico, Central America, and 
northwestern South America. In the southern part of its breeding 
range it is nesting in April, but the summer residents of New Bruns- 
wick do not reach their breeding grounds before the middle of May. 
Therefore, about 50 days are required for these northbound birds to 
B4504M 
FIGURE 3.—Isochronal migration lines of the black-and-white warbler, showing a slow and uniform migra- 
tion, the advance across the United States being apparently only about 20 miles a day. 
another 50-day trip south, and the earliest migrants from the northern 
areas would reach the Gulf coast in September. But both adults and 
young have been observed at Key West, Fla., by the middle of July, 
and on the northern coast of South America by August 21. Since 
the birds at Key West were fully 500 miles south of the breeding 
range, it is evident that they must have come from the southern part 
of the nesting area. 
Many similar cases might be mentioned, such as the black-throated 
blue warblers (Dendroica caerulescens), which are still observed in 
