BIRD STUDY 35 
Some think that the ice age crowded the birds southward 
from their ancient home in the Arctic Regions, which at one 
time had a temperate climate. Geological changes, such as the 
upheaval of mountain chains, the rise or subsidence of large 
areas of land, doubtless enter into the problem; evidently al} 
migratory routes are very ancient in their general outlines. 
Another question that may not be answered with too 
great assurance is, Why do birds migrate at all? What has al- 
ready been said suggests an ancient and long standing habit, 
which doubtless has something to do with migration. The cold 
of our winters is probably not so large a factor in migration as 
it is usually supposed to be. All birds are very warm blooded, 
and many of them endure our coldest winters without undue 
hardship unless caught in some exceptional storm or heavy 
snowfall. A few birds which usually go south in autumn fre- 
quently remain during the winter if certain food is plentiful. In 
South Dakota the writer has seen Western Mourning Doves 
and Western Meadowlarks in January, and Blackbirds have been 
seen feeding in cattleyards in the middle of our coldest winters. 
Robins, Bluebirds and Northern Flickers are frequently seen 
at Vermillion in winter, especially when wild grapes are abun- 
dant. A few years ago a flock of thirty or forty Robins was 
seen on the University campus and in adjoining ravines in mid- 
winter. 
The supply of food would seem to be a much larger factor 
than the need of warmth. In many cases the character of the 
food eaten would seem to affect the distance of migration. All 
insect eating birds, such as Swallows, Swifts, Flycatchers, Vireos 
and Warblers, must leave our winters and find summer lands 
where insects abound, which of course they do by going to the 
Southern Hemisphere. Snipes, Rails and Woodcocks, which 
gather their food around open water or in moist places, cannot 
live where everything is frozen. Birds that live on both insect 
larve and fruit, like Flickers, Robins and Bluebirds, need not 
go as far south as wholly insectivorous birds. Flickers do not 
go far south and Robins and Bluebirds winter in our Southern 
States, while the insectivorous birds push on for the most part 
into Central and South America. ; 
