NOCTURNAL MIGRATION 47 



statements by a somewhat uncertain belief that 

 flying became easier as altitude was gained, and 

 increased in difficulty as the earth was neared. Ex- 

 perience of aviators, however, is directly opposed to 

 this theory, as it is found that with greater altitude 

 there is increasing difficulty in maintaining height 

 and speed. Though part of this difficulty may be 

 due to the changing mixture offered for combustion 

 in the motor, more of it unquestionably comes from 

 the difference in buoyancy as the air becomes more 

 and more rarefied. Birds are not afflicted with the 

 engine troubles that beset humans who invade the 

 air, but must nevertheless feel keenly the lack of 

 buoyancy of the upper reaches. Though less effec- 

 tive in the case of broad-winged hawks, vultures, 

 pelicans, and cranes, whose great pinions afford a 

 large supporting surface compared to the weight of 

 the body, we must believe that the difference in- 

 dicated would operate more heavily in smaller 

 migrants, who are under necessity of keeping their 

 smaller, shorter wings in constant rapid motion to 

 hold a proper altitude even when flying near the 

 earth. 



Observations on the altitudes at which birds fly 

 have been summarized recently by Colonel R. 

 Meinertzhagen,^ who gives a considerable number of 

 records based on the experiences of airpilots. His 



I Ibis, 1920, pp. 920-936. 



