The Birds that Pass in the Night 



BY HARRY S. WARREN 



UPON the request of Mr. H. A. Winkenwerder, of 

 the University of Wisconsin, I made arrangements 

 to take observations with the telescope at Detroit 

 upon nocturnal bird migration, using the moon for a field 

 of vision, during the full moon in May. The moon would 

 be at full at eight a. m. on the 14th, Monday, but, 

 anticipating cloudy weather, we made our observations on 

 Sunday evening, the 13th, which was clear and warm, 

 with a very light southwest wind. As it was rather late in the 

 migrating season, we expected poor results, but a glance at the 

 tabulated figures below will show that we were pleasantly disap- 

 pointed. 



There were four principal facts we wished to establish by these 

 observations: the number of birds, the direction of flight, their 

 speed, and relative size. In order to obtain this data we numbered 

 the four cardinal points on the field of vision, and for every bird 

 that passed we wrote down, on blanks prepared for the purpose, 

 the point or fraction of a point at which he entered and that at 

 which he left the field, the relative speed at which he passed, and 

 the comparative size of the bird, as well as any further data observed 

 for each individual. The observations covered the time from 8.15 p. m., 

 which was the time the moon came clearly into vision, to 12 midnight, 

 and this time we divided into fifteen-minute periods so as to ascertain 

 the number of birds passing at any period of the evening, as shown 

 below. To make the work easier we changed watches every fifteen 

 minutes, one person using the telescope and the other writing down 

 the data. 



The telescope used was a 6-inch refracting instrument, equatorially 

 mounted, with an 8-foot focus; and the eye-piece, a forty-power 

 Clark lens. The number of birds passing during each period, their 

 directions of flight, their speed and comparative size are shown in 

 the following tables : 



Detroit, Michigan, May 13, 1900. 



^ / Min., 66° at 12 Midnight. 

 Atmosphere, fair ; wind, light, southwest. 

 Number of birds seen from 8.15 to 8.30 ... . 8 

 " " " " 8.30 to 8.45 .... 7 

 " " " " 8.45 to 9.00 .... 10 

 " " 900 to 9.15 .... 8 First hour 33 



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