56 THE MIGRATIONS OF BIRDS 



this line advances. The majority of species allow 

 spring to forge ahead, and then, travelling rapidly, 

 overtake it, or, in some cases, outstrip it. Hardy 

 forms among our smaller birds press northward with 

 the first indication of spring, so that robins appear 

 as soon as the ground is bare, and blackbirds when 

 the marshes open. Recurrent cold means a certain 

 amount of hardship, but has no other effect unless 

 too prolonged. Delicate species, of necessity, ad- 

 vance more slowly, as they must wait until spring 

 weather is settled. 



The northward flight of migrants in general is 

 initiated when wind and temperature are favorable. 

 If a sudden storm arises, the weaker migrants, if 

 over land, must descend to await better conditions. 

 If over water, they may be lost. Storms coming late 

 in spring often overtake the height of the small-bird 

 migration, and may entail much suffering, destitu- 

 tion, and loss of life. Yet such conditions are so 

 regularly met that most species pass through them 

 with discomfort but without particular danger, ex- 

 cept when they are unwontedly severe. On many 

 occasions snows during the first week in May greet 

 warblers and flycatchers in the northern states, and 

 for a day or two these small creatures may be seen 

 hopping about in unusual situations where there 

 may be a chance for food or shelter. Though many 

 may perish, usually the weather moderates before 



