112 THE FLIGHT OF BIRDS 



has still a stretch of more than 800 miles of sea to 

 fly over. Thus the New Zealand evidence, though 

 it is not yet as complete as we could wish, justifies 

 us in attributing to birds quite extraordinary powers 

 of endurance. 



I have already pointed out that the rarefaction 

 of the air at high altitudes makes it less resistant 

 and less buoyant, an advantage without any draw- 

 back. The migrant bird flying, say, some three, 

 four or five thousand feet above the sea level, will 

 not suffer from the attenuation of the atmosphere, 

 for, since he travels with very great velocity, it will 

 give him all the support he wants. But there is 

 another fact that must be borne in mind which may 

 make the achievements of migrant birds more 

 credible. 



Homing Pigeons, as we have seen, make the best 

 times as a rule when they have a tail-wind to help 

 them. Migrant birds, on the other hand, very often 

 have, it is said, the wind on the shoulder or blowing 

 almost straight in their faces when they make their 

 flights. But we must not without evidence jump to 

 the conclusion that the wind at the altitude at which 

 the birds are flying is blowing in the same direction 

 as it does at our level. We have only to watch the 

 clouds to discover that often at no very great height 

 there is an upper current that is not following the 

 same course as the wind below. Fifty years ago 

 Mr. Glaisher made some investigations, by means of 

 balloons, that threw much light on the subject. He 

 found that, though the direction of the wind close 

 to the earth was sometimes that of the whole mass 

 of air up to 20,000 feet, yet at other times the direc- 



