OCTOBER, 1902. 
THE MIGRATION OF lURDS. 
187 
During the past fifteen or twenty years much valuable work 
has been done on geographical distribution. Every student of 
migration must recognize the importance of a knowledge of the 
geographical distribution and dispersion of birds. The extended 
works of Baird, Allen, Merriam, Chapman, Hasbrouck and others 
on distribution, and the local bird lists that have appeared in re- 
cent years are invaluable as aids in the study of migration prob- 
lems. During the past twenty years a great advance has been 
made in work of this nature. Beside the many town and county 
lists that have appeared, there are state lists for many of the 
states of eastern North America. From the Bahamas, the We^t 
Indies and the northern portions of South America much infor- 
mation has been obtained on the distribution of birds that is in- 
valuable for the study of migration problems. 
Thus the problems of migration, after passing the period of 
superstitious beliefs, were next pursued for their immediate im- 
portance to climatological ends. At about the middle of the nine- 
teenth century naturalists turned them to the interests of zoology. 
The first point, then, was to establish the most advantageous 
course of study, which resulted in the national form of investiga- 
tion. During the past two decades, besides the many important 
results obtained, new fields and methods of investigation have 
been opened to the student, and at the present time individual 
observers and others united into societies are at work in nearly 
all parts of the world. 
CHAPTER II. 
THE CAUSES OF MIGRATION. 
We know that birds leave us in the fall and that they return 
again in the spring; we know something of their habits during 
migration, and we know of the changes in living nature concomi- 
tant with many of these flights. There are, however, so many in- 
fluences that might affect migratory movements that this subject 
is not easily handled. Whatever these influences are, we know 
that they affect different birds in different ways ; that they are 
operative over vast areas at the same time, and set countless num- 
bers of birds in motion simultaneously. In the fall migrations 
begin with the sunny days of July and continue even until the 
earth is covered with snow and ice. In the spring some of our 
migrants may be seen as early as the middle of February, while 
others are still on their northward way when the first have al- 
ready performed their parental duties. So closely do spring and 
fall movements approach each other that only a narrow interval 
