52 THE MIGRATION OF BIRDS 



spent a month in the autumn of 1901, he noticed 

 birds passing at heights varying from 20 to 200 feet, 

 all flying southwards. He concluded that " the 

 wind is certainly the main factor in migration 

 meteorology I am convinced that the direction of 

 the wind is, in itself, of no moment to the emigrants, 

 for they flitted across the Channel southwards with 

 winds from all quarters" (16). When the velocity 

 of the wind, however, was above 28 miles per hour 

 (a fresh breeze, force 5 on the Beaufort scale), no 

 migration was observed. 



Allowing this, and also taking into consideration 

 the undoubted fact that birds are frequently held 

 up by strong winds on the shore before starting 

 oversea journeys, is there any proof that they do 

 not actually avail themselves of fairly steady strong 

 winds when they reach the upper air ? 



Mr F. J. Stubbs makes some useful suggestions (50). 

 He points out that Gatke and others imagine that a 

 bird flying with the wind would suffer inconvenience 

 through the wind ruffling up its feathers. It is surely 

 evident that a bird supported in a moving medium 

 could not progress at any slower rate than that 

 medium ; the bird is not flying on one stratum of 

 air with another moving with a different speed 

 immediately above it ; it is actually in a current, 

 not on it. If the bird flies at 20 miles an hour, and 

 the wind or moving air is progressing at 10 miles an 



