52 THE MIGRATIONS OF BIRDS 



Ingram ^ has noted that normally the velocity of 

 the wind increases as one rises above the earth, so 

 that it may be doubled in strength at 1,500 feet. 

 Beyond this the amount of increase is lessened. In 

 his experience (apparently mainly in France) an 

 east wind attained its maximum strength at 3,000 

 feet, while winds from other quarters of the compass 

 may increase up to 30,000 feet. In ascending, the 

 directions of the winds also vary, so that they may 

 be reversed within a few hundred feet of altitude. 

 Both of the factors mentioned are of great impor- 

 tance in considering bird migration. 



We may conclude from the evidence that has been 

 presented that the bulk of migration passes below 

 3,000 feet, and that as a rule only birds of strong 

 flight travel at greater heights unless weather condi- 

 tions may, for brief periods, favor greater altitudes. 



Weather and Migration 



Cooke has well observed that weather conditions 

 have little to do with the migrations of birds, except 

 to permit them to travel or to hold them back, as the 

 case may be. Parula warblers or redstarts wintering 

 in the West Indies, or Old World warblers or wag- 

 tails spending the cold season in central Africa, 

 notice no difference in temperature or climatic con- 

 ditions between November and April, yet in the 



^ 7^/^,1919, p. 323. 



