2 BULLETIN 185, U. S. DEPARTMENT OF AGRICULTURE. 



showing how widespread is the recent development of this important 

 phase of nature study. 



The Survey has been collecting data on bird migration for more 

 than 25 years. Investigations by its field naturalists extending over 

 the North American Continent from Panama to the Arctic Circle have 

 resulted in voluminous notes, and in addition assistance of ornitholo- 

 gists throughout the country has been enlisted, so that each year 

 reports are received in spring and fall from hundreds of experienced 

 observers. Lighthouse keepers also have supplied valuable informa- 

 tion concerning the destruction of birds at their lights. The facts 

 gathered — over 500,000 — from these various sources form the largest 

 amount of data on bud migration ever collected in this country and 

 permit broader and safer generalizations than have hitherto been 

 possible. 



A knowledge of the times of migration of birds is essential as a 



basis for intelligent study of their economic relations and is equally 



necessary in formulating proper legislation for bird protection — two 



subjects which form important parts of the work of the Biological 



Survey. 



CAUSES OF MIGRATION. 



For more than 2,000 years the phenomena of bird migration have 

 been noted; but while the extent and course of the routes traversed 

 have of late become better known, no conclusive answer has been 

 found to the question, Why do North American birds migrate ? Two 

 different and indeed diametrically opposite theories have been ad- 

 vanced to account for the beginnings of these migrations. 



According to the more commonly accepted theory, ages ago the 

 United States and Canada swarmed with nonmigratory bird life, 

 long before the Arctic ice fields advancing south during the glacial 

 era rendered uninhabitable the northern half of the continent. 

 The birds' love of home influenced them to remain near the nesting 

 site until the approaching ice began for the first time to produce a 

 winter — that is, a period of inclement weather which so reduced the 

 food supply as to compel the birds to move or starve. As the ice 

 approached very gradually, now and then receding, these enforced 

 retreats and absences — at first only a short distance and for a brief 

 time — increased both in distance and duration until migration became 

 an integral part of the very being of the bird. In other words, the 

 formation of the habit of migration took place at the same time that 

 changing seasons in the year replaced the continuous semitropical 

 conditions of the preglacial eras. 



As the ice advanced southward the swing to the north in spring 

 migration was continually shortened and the fall retreat to a suitable 

 winter home correspondingly lengthened, until during the height of 

 the glacial period birds were for the most part confined to Middle and 



