PREFACE 



THE observations on the migration of birds pre- 

 sented in the following pages were delivered in 

 six lectures before the Lowell Institute in Boston on 

 October 26 and 28, and November 2, 4, 16 and 18, 

 1925. The manuscript as here published is un- 

 changed except for the inclusion of a few para- 

 graphs that time did not permit to be presented 

 during the course of the lectures. 



The material included is based on study and ob- 

 servation on the part of the author during a period 

 of more than twenty years. It is intended as a sum- 

 mary of present knowledge of migration, with the 

 various factors that affect it in its broader aspects, 

 without entering upon precise statistics of move- 

 ment for the many species involved. Such figures 

 may be left for treatises of another character. 



After careful consideration of the subject the 

 writer is profoundly impressed by the mass of detail 

 regarding the movements of birds that has been 

 assembled and the little that has been definitely 

 ascertained regarding the underlying principles that 

 control migration. There is much that remains to be 

 established in this phase of the subject. 



In conclusion the author desires to express appre- 

 ciation to the Bureau of Biological Survey, United 



