168 PROBLEMS OF BIRD LIFE. 



are set in motion at nearly the same time, and the members of 

 each flock or family, instead of flying in close order, scatter 

 about sufficiently to approach or mingle with the stragglers of 

 other flocks or families. Thus in effect they form a continuous 

 but straggling army, often hundreds of miles in length, and 

 varying in breadth according to the character of the country 

 over which it is passing. 



" Over a wide, level, and generally uniform region the host 

 spreads out in thin order ; following a river valley, it contracts 

 and thickens; and at narrow passes, such as the Straits of 

 Mackinac, it focusses its myriads into a solid stream. 



"As is well known, there are certain definite routes or paths 

 of migration along which birds pass in especially great num- 

 bers. These are usually coast lines, river valleys, or continuous 

 mountain ranges. Toward them converge innumerable less 

 frequented paths, each of which in turn has still smaller tribu- 

 taries of its own. Thus bird streams, like brooks, flow into 

 common channels, and each particular region may be said to 

 have its bird, as well as water, shed. An important consid- 

 eration is that the tributary bird streams follow courses in no 

 wise strictly dependent on points of the compass. 



" Bearing these facts in mind, the manner in which birds 

 find their way seems very simple. From the height at which 

 they fly the country presents the appearance of a map on 

 which, in the light of the moon or stars, the mountain ranges, 

 plains, lakes, rivers, and seacoasts are more or less distinctly 

 outlined for a hundred miles or more in any direction. (Any 

 one who has spent a clear night on the summit of a mountain, 

 will not question this statement.) Guided by such landmarks, 

 the older birds can have no difficulty in following paths which 

 they have repeatedly traversed before, and they unquestion- 

 ably direct and perhaps lead all the larger flights, although it 



