114 THE MIGRATIONS OF BIRDS 



riers of migration advance rapidly and cover a con- 

 siderable extent of country, perhaps reaching well 

 through the range long before the main army begins 

 to move. This is often true in fall flights where there 

 is no climatic bar to progress. Juncos appear in 

 middle latitudes in September, but do not become 

 abundant until the middle of October or the first 

 part of November. Yellow-legs start south in July, 

 and by the end of that month may reach South 

 America, though the main flight does not appear in 

 the extreme south until September or October. 

 Travel into the northern zones in spring, early in the 

 season, is retarded of course by the continuance of 

 cold in the north, so that the earlier movements 

 must await thaws. 



In early migration, there is much lingering along 

 the way to feed and rest, particularly among those 

 birds that travel long distances. In Argentina and 

 Uruguay I found migration northward among shore- 

 birds beginning at the end of January and well under 

 way in March, though the final rush did not come 

 until April. As this final flight coincides with the 

 initiation of migration among these birds in the 

 United States it will be seen that those who re- 

 sponded early to the migration impulse were slower 

 in progress than those that started later. 



Definite data for the rate at which birds travel 

 south in autumn are lacking, as at that season birds 



