86 THE MIGRATION OF BIRDS. 



by its marvellous power of memory and keenness 

 of sight. I would suggest, however, that the migra- 

 tion flight reaches its highest altitude wdien passing 

 over seas. These offer no landmarks, no bearings, 

 nothing that may serve as a guide ; consequently, 

 the line of flight rises to a sufficient altitude to 

 enable the bird to bridge the passage with its keen 

 powers of vision. In confirmation of this, I may 

 remark that in no part of the world do any regular 

 migration routes cross seas too wide to be bridged 

 by the eye of a bird flying at a sufficient altitude. 

 The am.ount of the earth's surface within the 

 compass of a bird's vision flying at any known 

 altitude is easily calculated by spherical geometry ; 

 and, putdng the average altitude at the very low- 

 estimate of from I GOO to 5000 feet, this will give 

 a wide enough area for all practical purposes. That 

 it is vastly exceeded, probably four- or five-fold, is 

 unquestionable. From this lofty course the earth 

 and sea for miles and miles wdll spread out below^ 

 these migrating birds in one unbroken panorama; 

 all the old familiar landmarks will be readily seen, 

 each peculiarity of coast, already known so w^ll by 

 experience, descried. All the dangerous or un- 

 suitable localities, all the favourite places of call, 

 wdiich years of former experience have taught them 

 to avoid or visit, will be displayed below them in 

 one broad and ever-changing expanse. This ten- 

 dency to a lofty flight amounts almost to a passion 

 as the time of migration approaches, and terrestrial 

 birds that have kept close to the bushes or the 



