THE MIGRATION OF NORTH AMERICAN BIRDS 65 
Differing routes to various parts of a large breeding or wintering 
ground, and used by large groups of individuals of other species, are 
not unknown. For example, the redhead duck is one of the common 
breeding ducks of the Bear River marshes of Utah, where a great 
many have been banded each summer. The recovery records of 
banded redheads show that while many travel westward to California, 
others start their fall migration in the opposite direction and, flying 
eastward across the Rocky Mountains, either turn southeast across 
the plains to the Gulf of Mexico, or deliberately proceed in a north- 
easterly direction to join the flocks of this species moving toward the 
Atlantic coast from the prairie regions of southern Canada. 
CONCLUSIONS 
The migration of birds as it is known today had its beginning in 
times so remote that its origins have been entirely obscured, and it 
can be interpreted now only in terms of present conditions. The 
causes underlying migration are exceedingly complex. The mystery 
that formerly cloaked the periodic travels of birds, however, has been 
largely dispelled through the fairly complete information that is now 
available concerning the extent and times of the seasonal journeys 
of most of the species. Many gaps, however, still remain in our 
knowledge of the subject. Much has been learned, and present 
knowledge is being placed on record, but it must be left to future 
study to clear away many of the uncertainties that continue to make 
bird migration one of the most fascinating subjects in the science of 
ornithology. 
Each kind of bird seems to have its own reaction to its environment, 
so that the character of movement differs widely in the various species, 
and seldom do any two present the same picture. In fact, bird mi- 
eration has been described as a phase of geographic distribution 
wherein there is a more or less regular seasonal shifting of the avian 
population caused by the same factors that determine the ranges of 
the sedentary species. If this view is correct, then it must be recog- 
nized that the far-reaching works of man in altering the natural con- 
dition of the earth’s surface can so change the environment necessary 
for the well being of the birds as to bring about changes in their yearly 
travels. The nature and extent of the changes wrought by man on 
the North American Continent are easily apparent. Forests have 
been extensively cut away and their places have been taken by second 
growth or cultivated land, and wide stretches of prairie and plain 
have been broken up, irrigated, and devoted to agriculture. These 
great changes are exerting a profound effect upon the native bird 
populations, and the various species may be either benefited or 
adversely affected thereby. 
The Federal Government has recognized its responsibility to the 
migratory birds under changing conditions brought about by man, 
and by enabling acts for carrying out treaty obligations, it is now 
givine many important species legal protection under regulations 
administered by the Bureau of Biological Survey. Much is being 
done by legislation for the welfare of the birds. The effectiveness of 
these conservation laws, however, is increased in the same measure 
that the people of the country become acquainted with the facts in the 
life histories of the migrants and interest themselves personally in 
