210 Carpenter, Height of Migrating Birds. [April 



AN ASTRONOMICAL DETERMINATION OF THE 

 HEIGHTS OF BIRDS DURING NOCTURNAL 

 MIGRATION. 



BY FREDERIC W. CARPENTER. 



Twenty-five years ago W. E. D. Scott, in the 'Bulletin of the 

 Nuttall Ornithological Club,' 1 called attention to the telescope as 

 a means for actually observing against the moon as a background 

 the nocturnal migratory flight of birds. Realizing the possibilities 

 of this method in throwing light on the unsettled question of ele- 

 vation, Mr. Scott, with the aid of Professor Young of the Astro- 

 nomical Department of Princeton University, made the following 

 observations and calculations. A nine and one-half inch telescope 

 was pointed toward the moon in the month of October, 1880. 

 Numbers of birds were seen flying in a general southeasterly direc- 

 tion. Since these birds were clearly outlined they must have been 

 at least one mile from the observers, for if nearer the telescope they 

 would have been out of focus. A distance of one mile was, there- 

 fore, taken as the inferior limit of the field of observation; and on 

 the assumption that birds would not fly at a greater height than 

 ten thousand feet the superior limit of the field was fixed at a 

 distance of four miles. These distances and the angle at which 

 the telescope was inclined gave the data for the conclusion that the 

 birds observed were flying from one-half 2 to two miles above the 

 earth. 



F. M. Chapman followed a few years later with a report in this 

 journal 3 of observations made with a six and one-half inch telescope 

 during a night in September, 1887. With the assistance of an 

 astronomical friend, John Tatlock, Jr., the writer prepared a table 



1 Scott, W. E. D.,'81. Some Observations on the Migration of Birds. Bull. 

 Nuttall Ornith. Club, Vol. VI, No. 2, pp. 97-100, 1 fig. Migration of Birds at Night. 

 Ibid., No. 3, p. 188. Note by J. A. Allen appended to each article. 



2 This lower limit is given by J. A. Allen and Newton and Gadow as one mile. 

 In the original paper Mr. Scott makes no actual statement of the dimension, but 

 in his figure (p. 100) the distance is represented by the perpendicular of a right 

 triangle, having an angle of 30° opposite, and an hypotenuse one mile in length, 

 This would make the perpendicular one-half mile long. 



3 Chapman, F. M., '88. Observations on the Nocturnal Migration of Birds. Auk, 

 Vol. V, No. 1, pp. 37-39. 



