Wetmore A. The migrations of birds

Cambridge, MA: Harvard University Press, 1926. — 252 p.The observations on the migration of birds presented 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 unchanged except for the inclusion of a few paragraphs that time did not permit to be presented during the course of the lectures.
The material included is based on study and observation on the part of the author during a period of more than twenty years. It is intended as a summary of present knowledge of migration, with the various factors that affect it in its broader aspects, without entering upon precise statistics of movement 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.