36 THE MIGRA TION OF BIRDS. 



tion to the winter quarters and then back again. As the one 

 movement is thus necessarily the complement of the other 

 it is hardly needful to seek for a separate cause for each, the 

 two together constituting migration in a complete sense. The 

 impulse to leave the breeding grounds may be prompted by 

 a reduction of temperature and a failing food-supply ; the 

 impulse to return may be more intimately associated with 

 the function of reproduction and the consequent necessity 

 of returning for this purpose to the proper home of the 

 species — to a region of peculiar conditions to which the 

 species has for long ages been undergoing special adapta- 

 tion. This is perhaps the best explanation we can give of 

 the origin and causes of migration. 



How birds find their way over the thousands of miles of 

 land and sea that often separate their winter and summer 

 homes has always been the subject of much speculation. 

 Until recently this wonderful power has been attributed to 

 " instinct." This is a way of saying that the matter is in- 

 volved in mystery, but also implying a sort of supernatural 

 power on the part of the migratory bird. As, however, the 

 facts of migration have become year by year better known 

 the subject has lost much of its former obscurity. It is now 

 known that birds migrate mostly by night, and as a rule at 

 great altitudes ; their sense of vision being acute, they are 

 thus able to discern for long distances the more prominent 

 features of the landscape — the coast lines, the larger rivers, 

 and the more prominent mountain chains, with which their 

 principal routes are found in a measure to coincide. Further- 

 more, birds migrate in large numbers at the same time, those 

 of different species becoming mingled and moving in loose 

 straggling parties ; in this way the individuals of a given 

 species may be always within sight or hearing of other 

 members of their own kin or of the general concourse ; it 

 being the habit of most birds while migrating to utter their 

 peculiar call-notes at frequent intervals. 



It is claimed by some observers that in the fall migration 



