70 75 80 85 



85 80 75 70 65 



Auslra] 

 Region 



Troiiic.il 



'rop>'-<-U T-ropicaJ 

 Region -I 



ZONE MAP OF NORTH AMERICA 



After U. S. Biological Survey 



1000 



statute Miles 



The dotted parts of the Aitslml Zones east of the Great Plains indicate the 

 li/ient of i/ie huynid divisions ijf these Zones.hnown ivspectiveli/as tlie AUeghanian, . 

 Carolinian and Austroripaiian Faunas. Ttie imdoUed ports nf the same zones 

 OTf knoTim as the Transition. Upper Sono7xtn and Lower Sonoran. 



MAP SHOWING THK BIRD ZONES 



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

 widely distributed than any 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. Tlie 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 ([uantity of heat during the season of growth and reproduction." Second, "Animals and 

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

 covering the hottest part of the year." With respect to birds the reference in botli instances 

 is to the breeding range. 



