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 all 

 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 

 larvae 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. 



