98 THE MIGRATION OF BIRDS 



But even putting aside the high altitude idea, and 

 confining our route-tracing to the known courses of 

 air currents, we shall find immense difficulty in 

 mapping out the actual course of any bird on any 

 particular day. The study of some of the publica- 

 tions of the Meteorological Committee, such, for in- 

 stance, as the " Life History of Surface Air Currents/' 

 by Shaw and Lempfert, published in 1906, shows the 

 great variation in the pathways, speeds, and forma- 

 tion of these systems ; a bird which accidentally 

 entered a cyclone would unconsciously alter its 

 actual track and speed very many times before it 

 passed beyond the area of influence. 



I am indebted to Mr Stubbs and Mr Herbert 

 Taylor of King's College, London, for some in- 

 teresting mathematically worked-out routes of birds, 

 travelling at a given speed in a cyclone rotating at 

 given speeds and moving at a fixed rate ; these show 

 great variation both in direction and speed according 

 to the time and place of entering the system. The 

 track of the bird is, of course, influenced by its own 

 rate of progress, by the speed of the rotating currents, 

 and by the rate at which the whole system moves 

 in any direction. Thus a migrant passing south 

 and coming within the influence of a cyclone which 

 is moving north-east at a high rate of speed, sa}^ 40 

 miles per hour, will, if it enters towards the northern 

 limits of the system, be at first retarded by the con- 



