DISPERSAL AND MIGRATIONS 171 



especially is undertaken by the sexes in company, 

 male and female arriving together. 



The duration of the migration of each species 

 is from one to two months at each season, having 

 regular phases of intensity. In some few species 

 the flight continues over a much longer time. 

 First the migration of a species is marked by 

 the arrival or departure of a few stragglers, the 

 most restless individuals ; then the migration of 

 that species becomes more pronounced, until its 

 period of greatest strength is reached, possibly 

 marked by one or two great "rushes," or extra- 

 ordinary numbers of individuals ; after which it 

 gradually dies away similarly as it commenced. 

 These phases of intensity are more characteristic 

 of some species than others. The whole period 

 that a migration of any one species lasts, how- 

 ever, is not occupied by absolute flight. The 

 time is taken up in gradually draining the 

 summer or winter area, and is of course shorter 

 in the species with a small distribution than it 

 is in those occupying a vast tract of country. 

 In some cases the breeding area of a species occu- 

 pies more than three thousand miles of latitude, 

 with a consequently great variation of climate 

 and date of season ; then it will be prolonged 

 over several months at least, the individuals 



