70 THE MIGRATION OF BIRDS 



Each species presents a separate problem, to be 

 solved for the most part only by patient, pains- 

 taking observation and by the recognition of sub- 

 species." 



The order in spring is yet unproved. " With many 

 birds . . . the first individuals to appear in spring 

 at a given locality are supposed to be old birds that 

 nested there the previous year." These are followed 

 by those which nested a little farther north, followed 

 later by those whose homes are in the most northerly 

 part of the range. ' If, then, for any species, the 

 southern nesting birds lead the van in both fall and 

 spring migration, and the near guard in each case is 

 composed of northern breeding birds, it follows that 

 some time between October and April a transposal 

 of their relative positions occurs ; and that the more 

 southern birds pass over the more northern ones, 

 which delay their migration, knowing that winter 

 still holds sway in their summer dominions." It is 

 not known where this transposal takes place, nor 

 whether the northern birds remain in winter quarters 

 till the southern birds have passed, or start a 

 slow migration, during which the southern birds 

 pass over them. Later another transposal occurs ; 

 the northern birds cross the southern part of 

 the range, passing birds which are already 

 nesting. " Spring migration seems to be there- 

 fore for some species a game of leapfrog the 



