194 THE MIGRATION OF BIRDS. 



We now proceed to discuss the Nearctic region. 

 Owing to prevalence of a much more severe climate 

 throughout the northern portion of this vast land 

 area, the comparative percentage of migratory birds 

 is considerably higher than in the Palasarctic region. 

 Although the land surface of the Nearctic region is^ 

 broadly speaking, only about half that of the Palas- 

 arctic region, the surface exposed to an Arctic 

 climate (say with a midsummer mean temperature 

 of 60° or less) is certainly more than double the 

 surface exposed to a similar temperature in the 

 Palaearctic region. This important fact is reflected 

 in the migrations of Nearctic birds. The great 

 majority of species breeding north of the United 

 States, or in about the same latitude as the south of 

 France and the valley of the Danube, are migratory ; 

 only the most boreal species remain to brave the 

 rigours of a British North American winter. On 

 the other hand, the summers are hot, and the 

 entire region supplies breeding-grounds to a very 

 large number of species. 



The first or winter zone of aquatic migratory 

 birds in the Nearctic region may be said to include 

 the Great Lakes and rivers of the United States, and 

 the St. Lawrence, down to the Gulf of Mexico in 

 the Atlantic, and the Californian Gulf in the Pacific. 

 The second or winter zone of land birds may be 

 said to include all but the most northerly of the 

 United States, Mexico, Central America, the West 

 Indies, and the northern portions of South America, 

 say to the Equator or the valley of the Amazon. 



