54 THE MIGRATION OF BIRDS 



Even in these days of upper-air investigation we 

 really know very little about the speed, direction 

 and steadiness of upper-air currents, but we do know 

 that at a moderate elevation some two or three 

 thousand feet the strength is usually greater than 

 nearer the earth. 



Mr Abel Chapman, in " Wild Norway," makes 

 this pertinent remark " Except by aid derived 

 from the operation of physical laws, the nature and 

 extent of which are unknown to me, and by taking 

 advantage of ' Trade- wind ' circulation in the upper 

 air, I believe that migration is impossible for short- 

 winged forms of sedentary habits but that aid, and 

 those advantages, may facilitate, and perhaps vastly 

 accelerate, a process which is otherwise impossible." 



In " Bird Life of the Borders " he goes further. 

 " Birds are warmer-blooded than ourselves or other 

 mammalia, and are capable of sustaining life in 

 rarified atmospheres where these could not. By a 

 simple mechanical ascent, they can reach, within a 

 league or two, regions and conditions quite beyond 

 human knowledge : where, selecting favouring air- 

 strata, they may be able to rest without exertion ; 

 or find meteorological or atmospheric forces that 

 mitigate or abolish the labours of ordinary flight, or 

 possibly assist their progress. ... It is in the upper 

 regions of open space where, I suggest, the final clue 

 will be found " (12). 



