Page:Bird-lore Vol 01.djvu/341

 The Birds that Pass in the Night BY HARRY S. WARREN PON the request of ]Ir. 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, INIonday, 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 pleasantl}' 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. rr ^ Max., 70° at 8 p. m. Temperature : -j j^j^^ 66° at 12 Midnight. Atmosphere, fair ; wind, light, southwest. . . 8 umber of birds seen from 8.15 to 8.30 " " " 8.30 to 8.45 " " " " ■ ' 8.45 to 9.00 " " " " 9.00 to 9-15 First hour 33 (113)