THE TRAVELS OF BIRDS 



the ten minutes he watched, a bird was seen to 

 pass slowly across his field. All were flying in 

 a southerly direction; but with the naked eye 

 not a bird could be seen. 



Our ears really tell us more than our unaided 

 eyes about the day flyers which are traveling far 

 up in the sky. With nothing to turn them from 

 their course, sound waves carry surprising dis- 

 tances either up from the earth or down to it. 



Balloonists tell us how clearly they can hear 

 voices of people who are almost indistinguish- 

 able. So we may hear the notes of passing birds 

 which are traveling at too great a height to be 

 seen. The mellow whistles of certain Snipe and 

 Plover tell us that they are passing on the birds' 

 air line when it is impossible for us to see them. 

 But if we answer we may in time see a black 

 speck in the sky, which responds to our call and 

 finally circles close overhead. 



On one occasion in Central Park, New York 



City, I hearol the flute-like call of a Yellow-leg 



Snipe which was migrating high over the city. 



Perhaps he was calling to some companion in 



72 



