396 GENERAL ZOOLOGY 



plover breeds in arctic America and winters in Patagonia. 

 The bobolink spends the summer in the northern part of the 

 United States and Canada but winters in Brazil. The paths 

 which the bobolinks follow in their two trips a year fall be- 

 tween the two broken lines shown on Fig. 209. By some 

 ornithologists the ability to travel these great distances is 

 ascribed to the possession of a sixth sense, — that of direc- 

 tion. In some cases rivers and coast lines are followed. It 

 has been observed that there are instances in which birds 

 now follow the outlines of old, submerged coast lines. 



While members of some species cover great distances in a 

 single day the migration speed is rarely more than fifty miles 

 an hour, and for all our species the average rate of migra- 

 tion is not much more than twenty miles a day, for there are 

 frequent stops of one or more whole days for feeding. 



The causes which underlie these great movements cannot 

 yet be stated with certainty. One very careful student of 

 bird migration has observed that in the spring migration the 

 appearance of greatest numbers of individuals and "first ar- 

 rivals" follows after a day of high temperature and winds 

 from the south when areas of low barometric pressure are 

 moving northward. It has been stated that the return to 

 the north is due to the desire of the birds to regain their old 

 home, or to their desire for seclusion during the breeding 

 season. The origin of the habit has been looked for in the 

 geological history of the world. During the Glacial Epoch 

 a great part of the northern hemisphere was shrouded in a 

 mass of ice, which came down from the north upon a region 

 which was then almost tropical in its climate. With the on- 

 ward advance of the ice, the birds, like all other forms of 

 life, were driven south, returning whenever the melting of 

 the ice permitted. Geology tells of many periods of alter- 

 nate progression and regression of the ice sheet, with ac- 

 companying changes of climate. It may well be that the 



