MIGRATORY FLIGHTS AT GREAT ALTITUDE. 297 



their migratory flights. Among others, the desire to 

 conceal their movements, and place themselves above 

 the usual haunts of birds of prey, and other enemies; 

 then again, the stiller, and cooler condition of the 

 higher atmosphere, and the fact that birds flying at 

 great altitudes can see to much greater distances 

 ahead, are doubtless all causes which influence their 

 movements. Mr. Dixon, in his valuable monograph on 

 the migration of birds, expresses himself strongly im- 

 pressed with this view, and says, 



" In my opinion the vast importance of altitude in migration 

 has never been recognised. Without considerable altitude 

 the migration of many birds would be simply impossible, and 

 that most birds fly unusally high during migration is, I think, 

 an unquestionable fact."* "The mere mechanical labour of 

 flight is rendered much easier of performance in the more 

 rarified atmosphere of these lofty regions of space. The 

 currents of air at great altitudes may also be more uniform 

 and favourable." f "The more I study migration," Mr. 

 Dixon proceeds to say, " the more I feel convinced that 

 there is a nocturnal drama in the air, of which only a faint 

 conception of its wonders can be formed by terrestrial scrutiny. 

 Many a time, the only sign on earth that a great migration 

 is in progress is by hearing the varied and often repeated 

 notes, sounding faintly from on high, as the armies of birds 

 pass on in the darkness overhead." 



The punctuality with which these periodical flights 

 are accomplished, almost upon the very same day, 

 each season as it comes round, is also one of their 

 most extraordinary and remarkable features; and we 

 need make no apology to our readers for transcribing 



The Migration of Birds, an Attempt to reduce Avian Season 

 Flight to Law, by Charles Dixon, 1892, p. 77, 78. 

 f Ibid., p. 83. 

 Ibid., p. 85. 



