isochronal lines. Fig. 2) Therefore, if 50 days are required to cross the 

 breeding range, and if 60 days are allowed for reproductive activities 

 and molting, they would not be ready to start southward before the 

 middle of July. Then with a return 50-day trip south, the earliest 

 migrants from the northern areas would reach the Gulf Coast in 

 September. Since adults and young have been observed on the 

 northern coast of South America by August 21, it is very likely that 

 they must have come from the southern part of the nesting area. 



Figure 1. Summer and winter homes of the black-and-white warbler. A very slow 

 migrant, these birds nesting in the northern part of the country take 50 days to cross 

 the breeding range. The speed of migration is shown in Fig. 2. 



18 



