DISTRIBUTION OF BIRDS 27 



acquaintance with birds without a formal attempt to add to our knowl- 

 edge of them. Bird study may not necessarily be anything so serious 

 as a study; it may be merely a recreation, a pastime, even a 'fad,' 

 if you like; but so long as our interest in birds is sufficient to take us 

 to their haunts, or so long as the voice of a bird expresses for us that 

 joy in nature which is the rightful heritage of every human being, 

 just so long will it repay us to add to our sources of pleasure that knowl- 

 edge of birds which will permit us to "come at these enchantments." 



THE DISTRIBUTION OF BIRDS 



Factors Influencing Distribution 

 , Faunal Areas 



Zones of the Boreal Region 

 Zones of the Austral Region 

 Floridian Fauna 



Factors Influencing Distribution. Possessed of a space-defying 

 means of locomotion, birds are more widely distributed than any other 

 vertebrates; but in spite of their unexcelled mobility their sensitive 

 organizations respond quickly to those influences which determine the 

 distribution of life. Consequently we find that while some species 

 have an atoiost world-wide range, others are confined to surprisingly 

 restricted areas. The factors determining the boundaries of the region 

 inhabited by any given bird may be classed primarily under the heads 

 of Past and Present. Past factors include those great earth-forming 

 forces which, through a series of profoundly important changes, have 

 brought about the now long-standing inter-relation of land areas in 

 other words, the world as we know it. The land bridges which connected 

 Great Britain with the Continent, or Alaska with Siberia, and the 

 strait which separated the American continents at Panama are factors 

 of this kind. In their time they obviously exercised a powerful influence 

 on the distribution of life. Were we equally sure of all the land con- 

 nections and water separations which have existed since life appeared 

 on the earth, we might hope to solve many at present inexplicable 

 problems in distribution. 



Of far-reaching importance also has been the evolution of climates 

 which this globe has witnessed, and which, through the last Glacial 

 Period, has introduced the climate under which we now live. Reference 

 to page 57 will explain how the distribution of White Pelicans, 

 for example, is believed to have been affected by such past climatic 

 changes. 



Climate, of course, has never ceased to exert its influence on the 

 distribution of life and we find it the most active present-day factor. 

 It is expressed mainly through temperature, and, to a lesser extent, 

 through rainfall. Thus the boundaries of the three transcontinental 

 zones forming the Austral Regipn (see cover map) conform more or 



