CONTENTS. 



Page. 



Introduction 379 



Migration waves and temperature waves 379 



Explanation of migration under apparently unfavorable conditions 381 



Migration and temperature 382 



Migration governed by average weather 383 



Birds prefer migrating in warm weather 384 



Birds can not foretell weather changes 385 



Beginnings of migration 386 



Uniformity of arrival 386 



Fall migration 387 



Migration and wind 387 



Equal flight lines 388 



Conclusion 390 



ILLUSTRATION. 



Page. 



FIG. 30. Northward migration of the summer warblers, compared with the 

 advance of spring 380 



