THE PHTLOSOPIIY OF MIGRATION. 83 



ing beacons to the birds flying on, and on. and on 

 above them ! 



We may allude to several other possible advan- 

 tages that may be gained by a lofty flight. The 

 mere mechanical labour of flight is rendered much 

 easier of performance in the more rarefied atmo- 

 sphere of these lofty regions of space. This is a 

 matter of great importance to species whose fly-lines 

 are very extensive, for they are thus able to fly 

 longer with less fatigue, and quiche?' than in the 

 lower and denser atmosphere — two things very 

 essential to the successful performance of their 

 journey. The currents of air at a great altitude 

 may also be more uniform and favourable, whilst 

 lower down, the air stream may be blowing in a 

 •direction quite unsuited to Flight. Another ad- 

 vantage of a lofty course is the greater immunity 

 from enemies, a very important item in migration, 

 as we have yet to learn. Far up in the sky, espe- 

 cially when shrouded in gloom, the way is free from 

 danger ; and small birds, even in daylight, are not 

 readily discerned at any great height above the 

 earth. One other advantage to diurnal migrants 

 especially is the lengthened period of daylight. 

 The valleys are hidden in gloom long after the 

 mountain tops continue to reflect the light of the 

 setting sun; and these lofty summits catch the dawn 

 long before it reaches the lower country. I have 

 already alluded to the possible use of the balloon 

 in studying Migration (see Idle Hours with Nature, 

 p. 217), and here I would seek to emphasize my 



