78 THE MIGRATION OF BIRDS. 



to say, that without a considerable altitude the 

 migrations of many birds would be simply im- 

 possible ; unless we were to attribute to these 

 creatures mysterious and supernatural powers of 

 perception, w^hich w^ould not only be most un- 

 scientific, but excessively absurd. That most birds 

 fly unusually high during migration is, I think, an 

 unquestionable fact. Witness the vast height to 

 w^hich Swallow^s and Swifts will soar just previous 

 to departure, or the startling suddenness with w^hich 

 migrating birds will drop perpendicularly from the 

 sky when their flight has been unexpectedly arrested 

 by meteorological influences. Other proofs are to 

 be found in the fact that migrants rarely strike 

 against lighthouses except during spells of sudden 

 darkness, due to fogs or clouds, which compel them 

 to seek a low^er altitude. Again, birds may be 

 actually observed migrating at vast heights. Gatke 

 records Rooks on passage flying so high that they 

 looked like dust, and were only recognized by their 

 cries. Of course, the size of large birds assists us 

 to distinguish them at an altitude of many thousands 

 of feet, especially those that habitually migrate by 

 day; but small birds may wing their way entirely 

 undetected at such enormous heights. Again, 

 migration, as w^e have already stated, is mostly 

 undertaken at night wdien birds cannot be seen at 

 all, although their cries may be repeatedly heard 

 obviously at a vast height as they wing their way 

 across the starry skies. As another instance of vast 

 altitude during migration, I may mention the fact 



