THE GEOGRAPHICAL DISTRIBUTION OF BIRDS. 



What do we mean by the "Geograph- 

 ical Distribution'" of birds? Are not 

 birds to be found everywhere, over both 

 land and sea? Are they not, then, uni- 

 versally distributed ? As a class they cer- 

 tainly are, but not as species nor even or- 

 ders. Parrots are not found in frigid re- 

 gions, nor are snowflakes and snowy owls 

 found in the tropical regions. Our Wood 

 Warblers and Vireos are not found out- 

 side of America, while there are no birds 

 of Paradise anywhere in America. We 

 shall see that most of the birds found in 

 the eastern hemisphere differ from those 

 found in the western, speaking broadly, 

 but that many of the island birds are dif- 

 ferent from birds of continents. 



Since most birds migrate shorter or 

 longer distances in search of a place to 

 rear their young, and return again to 

 warmer regions to pass the winter 

 months, the question at once arises, 

 What i9 the geographical distribution of 

 such migratory birds? That is not so 

 difficult as it may seem at first glance. We 

 have only to inquire what governs the 

 movements of the species in question in 

 such a way that its appearance at certain 

 places at certain known times may be 

 confidently expected. The study of mi- 

 gration and breeding has shown that the 

 impulse to move northward in the spring 

 to the old nesting-places where the young 

 are reared is more reliable than the im- 

 pulse to move southward on the approach 

 of cold. The birds are more certain to 

 appear at their old summer homes in 

 spring than they are to be found at any 

 particular place during the winter. But if 

 there be any objection to this view it will 

 yet remain true that where a bird rears its 

 young should more properly be called its 

 home than the place to which it is forced 

 by the approach of cold or the lack of 

 food. In either case, therefore, we may 

 regard the home of the bird, and there- 

 fore treat it9 distribution geographically 

 as the place where it habitually rears its 

 young. Having settled the question as 



to what shall determine the distribution 

 of the separate species, it remains to 

 study the physical conditions of the earth 

 for the sake of finding what it is that de- 

 termines the limits to which the different 

 species may go. 



We know that the distribution of land 

 and water over the earth has not always 

 been the same as it is now, but that many 

 places that are now covered with water 

 were once dry land, and that in many 

 places where there is now land there used 

 to be water. Now, America is wholly 

 separated from Uro-Asia-Africa, but 

 once they were connected together by a 

 broad neck of land where Bering Sea now 

 lies, and there may have been another 

 neck of land connecting Europe with Ice- 

 land and Greenland and so with North 

 America. Now Australia and New Zea- 

 land are wholly separated from all other 

 lands, but they were not so long ago. So 

 of the larger islands in general, they have 

 not always been isolated as now, but 

 connected with great land masses, shar- 

 ing with them the animals which roamed 

 over the whole vast regions. For in the 

 earlier times before Man had appeared 

 upon the earth, before the great Gla- 

 cial Period, the whole earth was tropical 

 in climate, making it possible for plants 

 as well as animals to live anywhere upon 

 the earth, as they cannot now. Then ex- 

 tensive migrations north and south were 

 not necessary, but instead there were 

 roamings about in all directions, or great 

 invasions of new regions by hosts of ani- 

 mals of one kind. 



As the land sank away here and there, 

 and the sea covered it, barriers were thus 

 formed to further roamings, except by 

 the birds of strong flight or animals that 

 could swim long distances, and there 

 could no longer be an intermingling of 

 the animals of the whole land surface of 

 the world. Since all animals are inclined 

 to change somewhat to meet or keep 

 pace with the changes that are going on 

 in vegetation and the general physical 



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