1920.] of the Migratory Flifjht of Birds. 929 



loudly up in the skies as lie was returning- from the coast to 

 Mount Ida. From where I was sitting I had a panoramic 

 view o£ a laroe j)art of southern Crete, and the Raven was 

 more or less level with me. It was some time before I could 

 pick him uj) with my glasses^ and ho then settled in a tree 

 some two miles from me. I see no reason why a flock of 

 Bee-eaters should not be heard at well over thut distance 

 when Hying overhead, but I very much doubt if they would 

 be visible to the naked eye at over two miles, and even witli 

 glasses I doubt the possibility of picking them up. We know 

 that sound travels better vertically down to the earth than 

 horizontally along it. An aeroplane at 17,000 feet is dis- 

 tinctly audilde, but would not be lieard at 3^ miles horizontal 

 distance. Now I never heard a Bee-eater call at night at a 

 greater distance from the earth than by day, which induces 

 me to believe that they do not pass at much over 3000 feet 

 by either day or night. If, according to Gaetke's theoi'v, they 

 were passing at about 20,000 feet, they would be invisible 

 and inaudible whether by day or night. 



Many other instances could be quoted showing that on 

 clear still nights — normal conditions — migratory flight is 

 clearly audible from the earth's surface, which tends to prove 

 that nocturnal migration is performed below 20,000 feet and 

 probably not much above 3000 feet. 



The thousands of birds which annually strike the lantern- 

 glasses of lighthouses do not constitute evidence of the 

 altitude of migratory flight, as this nearly always occurs 

 under abnormal conditions — fog, mist, rain, etc. — when all 

 migi-ation descends to near the earth's surface or sea-level. 

 I know of no instance of a migration " rush" at a lighthouse 

 in clear weather. 



The fact remains that migration '' rushes '' are rarely 

 witnessed except at lighthouses when unsuitable weather 

 conditions brings migrants down to a very low level, or when 

 migration has been checked or '' banked up " on account of 

 fog, rain, or storm. Though frequently seen in detail, 

 migration is but seldom witnessed in bulk. Masses of 

 migrants may be met with in a certain district and on a 



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