380 



YEARBOOK OF THE DEPARTMENT OF AGRICULTURE, 



Nashville warblers, the rarest of the regular migrants, were not un- 

 common. What had happened? A "bird wave" had met an un- 

 favorable " weather wave " and its progress had been temporarily 

 arrested. It was the height of the migration season, when untold 



.1 .I. Isotherms. 



Isochronal migration lines. 



J Main line of migration to the 



. Eastern United States. 



FIG. 30. Northward migration of the summer warblers^ compared with the advance of spring. 



thousands of aerial voyagers were speeding along the main chain of 

 the Alleghenies toward their summer homes. Suddenly they ware 

 met by a sleet storm of such severity as not only to block their 

 further northward progress, but to force them to descend from the 



