212 Carpenter, Height of Migrating Birds. [April 



of the birds with the size of prominent lunar features. Assuming 

 the average actual length of these birds (presumably small ones) 

 to be six inches, he was able to compute their distance from the 

 observer and their height above the sea-level, the latter proving to 

 be about two thousand feet. A larger bird, whose distance was 

 determined by focal adjustment, had an altitude of only 687 feet. 



In an extensive paper 1 on bird migration published in 1902, 

 H. A. Winkenwerder gives the result of work done by him- 

 self and several collaborators in different parts of the country. 

 At Beloit the telescope directed toward the moon's disk showed 

 birds which were apparently following the course of a river not far 

 from the observatory. It being taken for granted that these birds 

 were over the river it was an easy matter to determine their ap- 

 proximate heights. The majority were not over fifteen hundred 

 feet from the earth. The same conditions at Detroit gave evidence 

 that the flight was "somewhat more than one-half mile above the 

 surface. ' 



Two papers concerned with the height of birds in diurnal migra- 

 tion have come to my notice. In England R. A. Bray, 2 while look- 

 ing at the sun through an eight-inch telescope at three P. M. on 

 September 30, 1894, saw birds pass slowly across the field in a 

 southerly direction. The birds came one every few seconds for a 

 space of ten minutes. They were invisible to the unaided eye. 

 As both the birds and the sun were in focus the former must have 

 been, in the opinion of the writer, two or three miles away. 



The height as well as the velocity of a daytime flight of ducks 

 were accurately determined in December, 1896, by H. H. Clayton 3 

 at the Blue Hill Observatory, Massachusetts. This observer, 

 assisted by S. P. Fergusson, was engaged in measuring the heights 

 and velocities of clouds, making use of theodolites especially adapted 

 to these purposes. The appearance of a flock of ducks flying 

 southwest gave opportunity for applying the instruments to or- 



1 Winkenwerder, H. A., :02. The Migration of Birds with Special Reference to 

 Nocturnal Flight. Bull. Wis. Nat. Hist. Soc, N. S., Vol. II, No. 4, pp. 177-263, 

 pis. i-viii, 1 photo. 



2 Bray. R. A., '95. A Remarkable Flight of Birds. Nature, Vol. LII, No. 1348, 

 p. 415. 



3 Clayton, H. H., '97. The Velocity of a Flight of Ducks Obtained by Triang- 

 ulation. Science, N. S., Vol. V, No. 105, p. 26. The Height and the Velocity of 

 the Flight of a Flock of Geese Migrating: Northward. Ibid., No. 119, pp. 585-586. 



