because of the length of the days. Many warblers that normally fly at 

 night may find themselves over water at daybreak and be forced to 

 keep flying during the day until landfall is made. 



An interesting comparison of the flights of day and night migrants 

 may be made through a consideration of the spring migrations of the 

 blackpoll warbler and the cliff swallow. Both spend the winter as 

 neighbors in South America, but when the impulse comes to start 

 northward toward their respective breeding grounds, the warblers 



Isochronal Migration 

 Migration Route 



Figure 3. Migration of the blackpoll warbler. As the birds move northward, the iso- 

 chronal lines become farther apart, which indicates that the warblers move faster irith 

 the advance of spring. From April 30 to May 1 the average speed is about 30 miles per 

 day, while from May 25 to May 30 it increases to more than 200 miles. 



22 



