1^2 Libby, Nocturnal Flight of Migrating Birch. \_a£m 



looking the largest of the lakes in the vicinity of the city. The 

 writer was assisted by Winslow Mallery, to whose patience and 

 accuracy is due not a little of the success attending these initial 

 observations. It was proposed to watch the moon through a small 

 six-inch telescope, and to count the birds as they passed across 

 its surface in the southward flight. For convenience in keeping 

 the record, the whole time of observation was divided into periods 

 of fifteen minutes each and the count for each period kept dis- 

 tinct from the rest. The result exceeded all expectations and 

 well repaid the inconvenience attending such experimental work. 

 During the three nights of observation, Sept. n, 12 and 13, 1897, 

 a total of five hundred and eighty-three birds were counted, and 

 forty-five during one fifteen-minute period. On the evening of 

 the 12th, three hundred and fifty-eight were counted, the largest 

 number for any one period being thirty-five. The number of 

 birds seen during different hours of the night was very unequal. 

 The maximum number of three per minute was reached at 10.30, 

 and it diminished rapidly to a little more than one third of this 

 number at midnight. From this time the number declined, with 

 three considerable upward variations, to very near the zero point. 

 As to the direction of flight very great diversity was also observed. 

 The predominant direction up to ten o'clock was very nearly 

 south, and but comparatively few birds varied from this. The 

 diversity of direction, however, continued to increase till it 

 reached its maximum between twelve and two o'clock. At this 

 time the eight principal points of the compass were represented 

 by numbers varying from three to twenty-eight ; two-thirds of the 

 whole number still maintaining a southerly direction. 



The observations as to the number of birds and the direction 

 of their flight tell substantially the same story. The first con- 

 siderable falling off in the number of birds came at 11. 15, and 

 up to 10.45 tne y wer e observed to fly largely in one direction, 

 not half that number for any period taking any other direction. 

 Thus the intensity of the migratory movement, measured by the 

 number of birds and the regular direction of their flight, is seen 

 to be at its height early in the evening. The diminishing num- 

 bers and increasing variety in direction indicate plainly enough 

 that during the time of observation other things besides migration 



