NOCTURNAL MIGRATION 45 



tion in September, and estimates that birds passed 

 the point of observation at the rate of approximately 

 9,000 per hour — an indication of the enormous 

 extent of the nocturnal movement. 



Similar observations may be made by anyone at 

 the proper season by means of small telescopes or 

 powerful binoculars, though in my own experience 

 as a boy I found such instruments somewhat unsat- 

 isfactory, as the field of vision was so small that the 

 moving dots representing birds crossed the illumi- 

 nated area and disappeared with great rapidity. It 

 is possible to identify an occasional individual 

 among the birds that are detected; but efforts made 

 to estimate the height at which the birds are travel- 

 ling seem to me to yield highly uncertain results, as 

 there is no simple, definite method of determining 

 the actual distance between passing birds and the 

 point of observation in the brief space in which they 

 are on view. 



Records of nocturnal flight are made easily by the 

 ear, as many birds call during these flights by night. 

 During the rush at the height of migration these 

 notes, coming constantly from the darkness, pro- 

 duce a profound effect upon the imagination. Calls 

 come from near and far, some of them easily recog- 

 nized, and some so distant or so mingled with others 

 that they are indistinct. At times there may be a 

 medley in which half a dozen species may join, or 



