136 THE MIGRATIONS OF BIRDS 



river valley, mountain range, or other feature, but 

 such flight lines must not be visualized as narrow 

 paths or arterial traffic lanes, similar to the high- 

 ways of humans, designed especially to carry avian 

 travellers north and south; they are merely favored 

 passageways in one broad fly line that is continent 

 wide in extent. It is only at such places as Point 

 Pelee in Lake Erie, or Whitefish Point in Lake 

 Superior, that there is any true semblance to a 

 narrow lane. 



It must be borne in mind, then, in subsequent dis- 

 cussion that the lines of flight suggested are merely 

 broad lanes in which migration tends to concentrate, 

 or general tracts through which flight is particularly 

 abundant. Quite often we find that early arrivals 

 among birds are noted first at points in the concen- 

 trated lanes, and that only with the rush that marks 

 the passage of the mass of individuals of a species 

 do they appear at stations at the side. In some forms 

 the first bird will be seen in the course of some broad 

 north and south river valley, and it may be several 

 days before there is a spread to points at either side. 

 In other species movement may come on a broad 

 front extending indifferently across the land, so that 

 arrivals are noted simultaneously across the line of 

 advance. 



The accompanying diagram (see Fig. 5) to illus- 

 trate the author's conception of the main migration 



