BIRD MIGRATION 



By Wells W. Cdoke 



HE mystery of bird migration has proved a fascinating subject for speculation 

 and study from earliest times. Long ago it was noticed that birds disappeared 

 in fall and reappeared in spring, but, not knowing where they spent the 

 intervening period, many fanciful theories were advanced to account for 

 their disappearance, as hibernation in hollow trees or in the mud of streams 

 or ponds. With later years, however, has come a fuller knowledge of migra- 

 tion, especially of the particular region in which each species passes the cold 

 season, and more definite information in regard to the routes followed in 

 the spring and fall journeys. But fuller knowledge has served to increase 

 rather than to lessen interest in the subject. More persons to-day are 

 watching birds and noting their times of arrival and departure than ever before. 



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 jjroper legislation 

 for bird protection — two subjects which form important parts of the work of the United 

 States Biological Survey. 



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 advanced to account 

 for the beginnings of these migrations. 



According to the more commonly accepted theory, ages ago the United States and 

 Canada swarmed with non-migratory 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 to 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 in dura- 

 tion 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 semi-tropical conditions of the preglacial eras. 



As the ice advanced southward the swing to the north in the spring migration was con- 

 tinually 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 South America. But the habit of migration had been formed, and when the ice receded 

 toward its present position the birds followed it northward and in time established their 

 present long and diversified migration routes. 



Those who thus argue that love of birthplace is the actuating impulse to spring migra- 

 tion call attention to the seeming impatience of the earliest migrants. Ducks and Geese 

 push northward with the beginnings of open water so early, so far, and so fast that many 

 are caught by late storms and wander disconsolately over frozen ponds and rivers, prefer- 



