Vol. XXIII 

 1906 



Carpenter, Height of Migrating Birds. 



215 



point in question, i. e., with the distance of the bird from the 

 telescopes. 



With these chart records and the necessary astronomical data 

 Professor Stebbins was able to ascertain the "parallax" of each 

 bird, that is, the angle at the bird subtended by the two observers, 

 which is equal to the angular distance between the two lines of 

 flight appearing on the chart. Using this as a basis the distance 

 of the bird from the observers and its height above the ground ' 

 were computed. The probable direction of flight was also deter- 

 mined, although in so doing it was necessary to assume that the 

 flight was horizontal, as it doubtless was in the majority of cases. 

 This assumption was not needed in calculating the heights. 



The accuracy of the measurements depended on the skill of 

 the observers in making their records and on the amount of "paral- 

 lax." In Professor Stebbins's opinion the computation of the 

 greatest height (5400 feet) involved a possible error of twenty-five 

 per cent., while the lesser heights were probably correct within ten 

 per cent. 



Observations were made on several nights in both spring and 

 autumn, the best success being met with on May 19-20, and Oc- 

 tober 10. On the first date 78 birds were seen during two and 

 one-half hours, 11 by both observers, 33 by Professor Stebbins only, 

 and 34 by the writer only. On October 10, in two hours, 57 birds 

 were counted, 11 being visible through both telescopes: of the 

 remainder Professor Stebbins saw 17 and the writer 29. 



From nine satisfactory chart records of each night Professor 

 Stebbins has prepared the material for the following tabular views. 



Table I. 

 Birds migrating on the night of May 19-20, 1905, one day after 

 full moon. Weather clear. Wind northwest, 4 miles an hour. 

 Temperature at 9 P. M. 55° F. 



