CHAPTER IV 



THE HEIGHT AND SPEED OF MIGRATION FLIGHT 



IN the last chapter reference was made to the great 

 height at which birds may fly on migration. Certain 

 species, even comparatively weak- winged ones, appear 

 normally to fly high, whilst others, often birds with 

 pointed wings and great aerial powers, usually proceed 

 at low elevations ; but there is still much conjecture 

 as to the actual altitude reached by any migrants. 



Gatke was of opinion that we do not see much of 

 real migration, which is certainly correct, but there 

 is no reason for his statement that it normally takes 

 place at anything like the altitudes he mentions, 

 30,000 feet or more, or that birds at the time of 

 migration undergo physiological changes which en- 

 able them to fly at immense heights and speeds and 

 to see clearly in the dark (29). Nor need much 

 weight be placed on the speculation of Lucanus, who 

 contends that the height of travel must be less than 

 1000 metres, for above that elevation aeronautical 

 observations show that perspective lessens. There 

 are actual observations which, though liable to a 

 margin of error, are proof of migratory flight at very- 

 high altitudes. 



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