38 THE MIGRATION OP BIRDS 



when, either on account of the indifferent food- 

 supply or of unsuitable weather conditions, the 

 speed is accelerated. 



In America Mr Cooke proves that the Mississippi 

 Valley is undoubtedly utilised as a fly-line by a 

 large number of species, but by no means all, and 

 his evidence, though proving the use of routes, is 

 that these are seldom constricted pathways but 

 broad areas crossed in a generally coincident direc- 

 tion by the birds which make use of them. This 

 main fly-line is however formed in America as in 

 other places by the convergence of subsidiary 

 streams, and it is these tributaries, as Herr Herman 

 points out, which have in many instances led to 

 error ; they have been mistaken for main routes. 

 The main route may be compared to the trunk of 

 a tree, the birds following the roots from the area 

 in which they have been nesting or wintering, 

 and at the end of the journey splitting off in various 

 directions, like the branches, to their temporary 

 winter or summer homes. 



The contrast in the method of travelling of 

 different species or of the same species under differ- 

 ent conditions, may be realised by taking two 

 examples. Firstly, Mr Eagle Clarke's experiences 

 at the Eddystone and Kentish Knock Lightship, 

 when birds passed during the daytime at varying 

 elevations, sometimes close to the waves, in twos or 



