el 
so gay 
nN ees 
35 
30 
Arctic zone 
Boreal /Hudsonian ee MASS 
e610 |canadian y ‘ iN SS 
2 iy MI 
Transition < 
erie Unperaustell (OC Wr \ 
fern LowerAustralf x 
Tropical\-y, 4: r 
Region \tr opical YH 
ZONE MAP OF NORTH AMERICA 
After U.S. Biological Survey 
fo} 
500 1000 SK a 
“a 
Statute Miles oy 
The dotted parts of the Austral Zones east of the Great Plains indicate the 
A 
Sb 
extent of the humid divisions of these Zones, knawn respectively as the Alleghanian, A 
Carolinian and Austroriparian Faunas. The undotted parts of the same zones 9 APE Se 
85 0 
are known as the Transition, Upper Sonoran and Lower Sonoran. 
10S 
hs 
120 us No 100 9s 90 75 70 A 4 BUNSTLAD 
MAP SHOWING THE 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. 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 year.” With respect tn hirds the reference in hath inctanene 
is to the breeding range. Z 
