chap, ii.] BIRDS. 17 



return along the eastern coasts of the Uuited States, and often 

 fly from point to point across bays and inlets. They are then 

 liable to be blown out to sea by storms, which are prevalent at 

 this season ; and it is almost always at this time of year that 

 their occurrence has been noted on the- shores of Europe. It 

 may, however, be doubted whether tins is not an altogether 

 modern phenomenon, dependent on the number of vessels con- 

 stantly on the Atlantic which afford resting-places to the wan- 

 derers ; as it is hardly conceivable that such birds as titlarks, 

 cuckoos, wrens, warblers, and rails, could remain on the wins 

 without food or rest, the time requisite to pass over 2,000 miles 

 of ocean. It is somewhat remarkable that no European birds 

 reach the American coast but a few which pass by way of 

 Iceland and Greenland ; whereas a considerable number do 

 reach the Azores, fully half way across; so that their absence 

 can hardly be due to the prevailing winds being westerly. The 

 case of the Azores is, however, an argument for the unassisted 

 passage of birds for that distance ; since two of the finches are 

 pecidiar ' species,' but closely allied to European forms, so that 

 their progenitors must, probably, have reached the islands before 

 the Atlantic was a commercial highway. 



Barriers to the Dispersal of Birds. — We have seen that, as a 

 rule, wide oceans are an almost absolute barrier to the passage of 

 most birds from one continent to another ; but much narrower 

 seas and straits are also very effectual barriers where the habits 

 of the birds are such as to preserve them from being carried 

 away by storms. All birds which frequent thickets and forests, 

 and which feed near or on the ground, are secure from such 

 accidents ; and they are also restricted in their range by the 

 extent of the forests they inhabit. In South America a large 

 number of the birds have their ranges determined by the ex- 

 tent of the forest country, while others are equally limited to the 

 open plains. Such species are also bounded by mountain ranges 

 whenever these rise above the woody region. Great rivers, such 

 as the Amazon, also limit the range of many birds, even when 

 there would seem to be no difficulty in their crossing them. The 

 supply of food, and the kind of vegetation, soil, and climate 



