The geographical distribution of animals, with a study of the relations ...

chap, ii.] BIRDS. 17return along the eastern coasts of the United States, and oftenfly from point to point across bays and inlets. They are thenliable to be blown out tosea by storms, which are prevalent atthis season ; and it is almost always at this time of year thattheir occurrence has been noted on the shores of Europe. Itmay, however, be doubted whether this is not an altogethermodern phenomenon, dependent on the number of vessels constantlyon the Atlantic which afford resting-places to the wanderers; as it is hardly conceivable that such birds as titlarks,cuckoos, wrens, warblers, and rails, could remain on the wingwithout food or rest, the time requisite to pass over 2,000 milesof ocean. It is somewhat remarkable that no European birdsreach the American coast but a few which pass by way ofIceland and Greenland ; whereas a considerable number doreach the Azores, fully half way across ; so that their absencecan hardly be due to the prevailing winds being westerly.Thecase of the Azores is, however, an argument for the unassistedpassage of birds for that distance ; since two of the finches arepeculiar ' species,' but closely allied to European forms, so thattheir progenitors must, probably, have reached the islands beforethe Atlantic was a commercial highway.Barriers to the Dispersal of Birds.—We have seen that, as arule, wide oceans are an almost absolute barrier to the passage ofmost birds from one continent to another ; but much narrowerseas and straits are also very effectualbarriers where the habitsof the birds are such as to preserve them from being carriedaway by storms. All birds which frequent thickets and forests,and which feed near or on the ground, are secure from suchaccidents ; and they are also restricted in their range by theextent of the forests they inhabit. In South America a largenumber of the birds have their ranges determined by the extentof the forest country, while others are equally limited to theopen plains.Such species are also bounded by mountain rangeswhenever these rise above the woody region.Great rivers, suchas the Amazon, also limit the range of many birds, even whenThethere would seem to be no difficulty in their crossing them.supply of food, and the kind of vegetation, soil, and climate