70 65 



Austral 

 Region 



Trc,,„calU i^^^, 



ZONE MAP OF NORTH AMERK A 

 Afli?r U. S. Bi( lo£itnl Surve> 



Staluip Miles 



The dotted parts of fjie Au^^ifulZoriev ea^l <j/' t/if Ltrfnt Ptainfi indicate itir "- 

 edient qftiie tiumiddiimifms ff these Zotws. tcnown re^pectiveli/ aji i}ie AUeglianian, 

 Caroti nian and AuxtroTiparian Faunae. Ttie uridatied parts of tAe same zones 

 OTe knonni as //w Transition, Upper So^iOTun and Lower Sonoran . 



MAP SHOWING THE BIRD ZONES 



With their unparalleled facilities for locomotion over both land and water, birds are more 

 widely distributed than anj' other vertebrates, yet their comparatively delicate organizations 

 are highly sensitive to many conditions of life. The most important factor in determining 

 the breeding range of birds is, of course, the one of climate, expressed mainly through tem- 

 perature and to a lesser degree through rainfall. 



The zone map shows that North America is divided into three great regions — the Boreal, 

 Austral, and Tropical — and the first two in turn are subdivided into three zones each. The 

 boundaries of the zones follow in a general way certain isotherms, or lines of temperature, 

 with subdivisions determined by the annual rainfall. The numerous "lakes" and "islands" 

 noted on the zone map are brought about in most cases by the altitude of these areas. 



In connection with this map it is well to keep in mind C. Hart Merriam's Laws of Tem- 

 perature Control : First, "Animals and plants are restricted in northward distribution by the 

 total quantity of heat during the season of growth and reproduction." Second, "Animals and 

 plants are restricted southward in distribution by the mean temperature of a brief period 

 covering the hottest part of the \ear." With respect to birds the reference in both instances 

 is to the brccdiiiq range. 



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