REGULARITY OF MIGRATION 77 



In general in spring migration there are two main 

 periods of abundance in the central portions of the 

 United States, the first of which is in early spring 

 as soon as snow leaves the ground. Crowds of hardy 

 migrants arrive, and hordes of species uncommon 

 in winter pass from more southern regions. In in- 

 land areas the period is one of abundance for finches 

 and sparrows. As spring opens in full, and the 

 weather moderates at more northern points, the 

 crest of this early wave passes, leaving summer resi- 

 dents to seek their breeding grounds and later ar- 

 rivals to straggle through. The first week or two of 

 April often represents a slack period, when bird-life 

 seems scant after the previous abundance, though 

 arrivals of new species are noted constantly. These 

 later migrants increase steadily until about the 

 middle of May, when the great seasonal rush of the 

 year is noted, with the passing of warblers, fly- 

 catchers, vireos, and many others; and it is then that 

 we hope to make our largest lists of species, since 

 early laggards and late arrivals mingle in the great- 

 est abundance of the year. By May 20 this rush is 

 abated, and by the end of May birds are on their 

 breeding grounds and belated migrants are few in 

 number. 



The autumn migration pursues a more even 

 course. The earliest arrivals among northern shore- 

 birds are noted on mud-bars or sandy beaches dur- 



