76 THE MIGRATIONS OF BIRDS 



of trees, for what food they may find. Near the 

 northern limit of their range deep snows periodically 

 reduce them almost to extinction, yet they do not 

 attempt a southward flight to escape. 



Abundance of birds during migration is also a 

 point of considerable interest. Observers record two 

 distinct stages in the movements of migratory birds 

 on their return north in spring, the date of first ar- 

 rival, when possibly a single individual may be seen 

 in some particularly favored haunt, and the date 

 when the species becomes common, which represents 

 a period when seasonal conditions become generally 

 favorable. Some birds arrive in numbers when first 

 noted, but usually the date of bulk arrival is deferred 

 for a few days or a week, or, rarely, for as long as a 

 month after the first have been seen. With birds 

 that do not nest at the point of observation the 

 period of bulk arrival is followed by a time during 

 which the species is seen regularly; and then sud- 

 denly the majority disappear and only stragglers 

 may be found until finally the last of these are gone. 

 Counting early and late stragglers, the migration 

 thus may extend over a period of two, or even three 

 months, but its main part is usually crowded within 

 ten to twenty days at the time when conditions are 

 most favorable. The length of time of possible oc- 

 currence, of course, decreases steadily in species of 

 late average arrival. 



