i 
4 
He tel 
i a Hud 
hit se ssh tgit are 
f + Sip) th ee 
> Lah i i 7 
= 
ew 
at 
AINA S 
Arctic zone ; Zi: as 
Boreal }Hudsonian + <> 
Region Canadian a 
‘Auseal Transition = é 
pee UpperAustral UU {] 
een LowerAustra] Rss 
Tropical)-,,.: r 
opical 
Region }Propic ; 
ZONE MAP OF NORTH AMERICA Sw KOK | Sepa 
After U.S. Biological Survey A ERE SOT ZZ BE SEA 
a 
—— 
1000 | Ws CARIBBEAN 
—— > —— SS = — 
Statute Miles 
Carolinian and Austroriparian Faunas. The undotted parts of the same zones 
are known as the Transition, Upper Sonoran and Lower Sonoran. 
120 us 110 05 100 95 90 
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 to birds the reference in both instances 
is to the breeding range. 
6 
