4 
THE MIGRATION OF NORTH AMERICAN BIRDS 3) 
in the year, the migrants are assured of adequate space and ample 
food wpon their arrival in the winter-freed North, and it may be 
assumed that nonmigrant species resident in the South are benefited 
by the withdrawal of the migrants. 
Nevertheless it cannot be said that the winter or summer areas are 
entirely unsuited to the requirements of every migrating species at 
other seasons, for some individuals pass the winter in areas that are 
frequented only in summer by other individuals of their species. The 
extensive breeding ranges of such species present wide climatic va- 
riations, so that some individuals may actually be resident in a region 
where others of their kind are present only in winter. 
The tendency in many species to move southward at the approach 
of winter is not always due to the seasonal low temperatures, since 
experiments have demonstrated that many summer insect feeders, 
when confined in outdoor aviaries, comfortably withstand tempera- 
tures far below zero. ‘The main consideration is the depletion of the 
food supply, caused either by disappearance or hibernation of insects, 
or by the mantle of snow or ice that prevents access to the seeds and 
other forms of food found on or close to the ground or submerged in 
water. Possibly also the shortened hours of daylight materially 
restrict the ability of the birds to obtain sufficient food at a time when 
the cold requires an increased supply to maintain body heat. It is 
noteworthy that chickadees (Penthestes atricapillus) and some other 
of our smaller birds have no fear of Arctic weather, as their food sup- 
plies are mainly arboreal and so are always available. Also, when 
there is a good supply of food in the form of pine seeds in Canadian 
woods, nuthatches (Sitta carolinensis and S. canadensis) and crossbills 
(Loria curvirostra and L. leucoptera) will remain through the winter. 
When these birds appear abundantly in winter at points in southern 
latitudes, it may be concluded that there is a shortage of their food in 
the North, or that they have been lured farther south by the greater 
abundance of this food there. 
THEORIES OF THE CAUSES OF MIGRATION 
Migration has long since become a definite hereditary habit that 
recurs in annual cycles, probably because of physiological stimuli 
associated with the reproductive period. In seeking its origin it is 
necessary to study the history of the birds’ occupation of their present 
ranges, and from the information available to consider what appear 
to be reasonable theories. The two now most commonly accepted 
are diametrically opposed to each other. 
NORTHERN ANCESTRAL HOME THEORY 
According to one of these, nonmigratory birds swarmed over the 
entire Northern Hemisphere in earlier ages, the conditions of food and 
habitat being such as to permit them to remain in their haunts through- 
out the year. The entire northern area then afforded the two impor- 
tant avian requirements—suitable breeding conditions and a year- 
long food supply. This is the condition today in the Tropics, and it 
is noteworthy that many tropical birds are nonmigratory. Gradually, 
however, in the Northern Hemisphere the glacial ice fields advanced 
southward, forcing the birds before them, until finally all bird life was 
concentrated in southern latitudes. As the ages passed and the ice 
