THE rillLOSOniY OF MIG RATION. 75 



The Migration of most, if not all, species may 

 be roughly calculated at from one to two months 

 each way, and has its regular phases of intensity. 

 In some nomadic species it lasts much longer, 

 as we shall learn in a future chapter. First, the 

 migration of a species is marked by the advent or 

 departure of a few stragglers, the most venturesome 

 and restless individuals ; then the flight becomes 

 more vast until the culminating point is reached, 

 perhaps in two marked rushes, after which it ebbs 

 pretty much as gradually as it flowed. The phases 

 of intensity are more marked and sharply defined 

 in some species than in others, and are due probably 

 to various local causes. It must not be supposed, 

 however, that the period during which the migra- 

 tion of a species lasts, is actually taken up in direct 

 flight. The time is taken up in draining the 

 summer or winter area, and is longer or shorter 

 according to the width of latitude cleared. Where 

 the breeding area of a species extends over 3000 

 miles of latitude, with its consequent great vari- 

 ation of climate and date of season, the migration 

 will be prolonged ; birds starting from the most 

 northern localities months before they do from the 

 more southern ones. Where the breeding area of 

 a species is narrow the n:igration is short, because 

 most of the birds start at the same time. It. is the 

 same with the winter quarters. Where they extend 

 far to the south they are filled more slowly than 

 where they are restricted to narrower zones. Tiie 

 evidence that birds leave their summer or winter 



