THE MIGRATION OF BIRDS 



ately dash upward and away again. Not till dark 

 do they finally settle to roost. Until late at night a 

 great chorus of voices may be heard among the 

 branches. The multitude increases daily for six 

 or eight weeks, additions, in the form of new family 

 groups, constantly augmenting their numbers. Some 

 time in September the migration call reaches the 

 Martins, and, yielding to its spell, they at once depart 

 toward their winter home in tropical South America. 

 The Usual Movement. — Many of our smaller birds, 

 such as Warblers and Vireos, do not possess a strong 

 flocking instinct, but, nevertheless, they may be seen 

 associated in numbers during the season of the north- 

 ern and southern movements. Such birds migrate 

 chiefly at night and have been observed through 

 telescopes at high altitudes. Such observations are 

 made by pointing the telescope at the disk of the full 

 moon on clear nights. On cloudy or foggy nights 

 the birds fly lower, as may be known by the clearer 

 sounds of their calls as they pass over; at times one 

 may even hear the flutter of their wings. There is a 

 [67] 



