46 THE MIGRATIONS OF BIRDS 



again a band of some particular form may pass with 

 their cries coming from all points of the air, to the 

 exclusion of others. The impression given at times is 

 so vivid that instinctively one strains the eyes 

 against the darkness in a vain attempt to pick out 

 the migrants whose vibrant calls come from so near 

 at hand that the birds seem almost within reach. 



Though migration flights at times are recorded 

 at lighthouses, and by means of call notes, steadily 

 all night long, usually the bulk of birds pass during 

 the earlier hours of the night and toward day-break 

 in the morning. From eight to twelve in the even- 

 ing seems to be the favorite period for nocturnal 

 flight, though on a number of occasions particularly 

 in autumn, I have noticed heavy migration between 

 four and six in the morning. 



Altitudes at which Migrating Birds Travel 



Actual evidence of the heights above the earth 

 sought by birds during their migrations is scanty, 

 and only since the development of the airplane have 

 definite altitudinal observations become available. 

 Early records have been based largely on inference, 

 or on calculations into which entered a wide margin 

 of estimate. 



The older observers, holding firmly that most 

 normal migration took place at heights above 15,000 

 feet, attempted as a rule to justify their faith in these 



