36 THE MIGRATION 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 
speciés—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 isa 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. Itis 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 
