THE GEOGRAPHICAL DISTRIBUTION OF BIRDS. 



What do we mean by the "Geograph- to what shall determine the distribution 



ical Distribution" of birds? Are not of the separate species, it remains to 



birds to be found everywhere, over both study the physical conditions of the earth 



land and sea? Are they not, then, uni- -for the sake of finding what it is that de- 



versally distributed ? As a class they cer- termines the limits to which the different 



tainly are, but not as species nor even or- species may go. 



ders. Parrots are not found in frigid re- We know that the distribution of land 



gions,norare snowflakes and snowy owls and water over the earth has not always 



found in the tropical regions. Our Wood been the same as it is now, but that many 



Warblers and Vireos are not found out- places that are now covered with water 



side of America, while there are no birds were once dry land, and that in many 



of Paradise anywhere in America. We places where there is now land there used 



shall see that most of the birds found in to be water. Now, America is wholly 



the eastern hemisphere differ from those separated from Uro-Asia-Africa, but 



found in the western, speaking broadly, once they were connected together by a 



but that many of the island birds are dif- broad neck of land where Bering Sea now 



ferent from birds of continents. lies, and there may have been another 



Since most birds migrate shorter or neck of land connecting Europe with Ice- 

 longer distances in search of a place to land and Greenland and so with North 

 rear their young, and return again to America. Now Australia and New Zea- 

 warmer regions to pass the winter land are wholly separated from all other 

 months, the question at once arises, lands, but they were not so long ago. So 

 What is the geographical distribution of of the larger islands in general, they have 

 such migratory birds? That is not so not always been isolated as now, but 

 difficult as it may seem at first glance. We connected with great land masses, shar- 

 have only to inquire what governs the ing with them the animals which roamed 

 movements of the species in question in over the whole vast regions. For in the 

 such a way that its appearance at certain earlier times before Man had appeared 

 places at certain known times may be upon the earth, before the great Gla- 

 confidently expected. The study of mi- cial Period, the whole earth was tropical 

 gration and breeding has shown that the in climate, making it possible for plants 

 impulse to move northward in the spring as well as animals to live anywhere upon 

 to the old nesting-places where the young the earth, as they cannot now. Then ex- 

 are reared is more reliable than- the im- tensive migrations north and south were 

 pulse to move southward on the approach not necessary, but instead there were 

 of cold. The birds are more certain to roamings about in all directions, or great 

 appear at their old summer homes in invasions of new regions by hosts of ani- 

 spring than they are to be found at any mals of one kind. 



particular place during the winter. But if As the land sank away here and there, 



there be any objection to this view it will and the sea covered it, barriers were thus 



yet remain true that where a bird rears its formed to further roamings, except by 



young should more properly be called its the birds of strong flight or animals that 



home than the place to which it is forced could swim long distances, and there 



by the approach of cold or the lack of could no longer be an intermingling of 



food. In either case, therefore, we may the animals of the whole land surface of 



regard the home of the bird, and there- the world. Since all animals are inclined 



fore treat its distribution geographically to change somewhat to meet or keep 



as the place where it habitually rears its pace with the changes that are going on 



young. Having settled the question as in vegetation and the general physical 



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