[31] BIRDS OF OREGON 



disturbed by many local factors of configuration alone. The higher or 

 colder zones tend to extend downward into the canyons with the flow of 

 cold air, while the plants of the warmer zones tend to creep upward along 

 the south side of the ridges and thus take advantage of the better air 

 drainage and the southern exposure. This fact is regularly used to advan- 

 tage by farmers in planting fruits and nuts in Oregon where, everything 

 else being equal, air drainage is the factor that furnishes protection 

 against innumerable frosts and determines success or failure of an orchard 

 section. 



This brief discussion of the zone concept as used by modern biologists 

 must suffice here, without an attempt to go further into the many often 

 obscure environmental factors that affect and disrupt the zones. The 

 following life zones are recognized in North America: 



i. Tropical 



2.. Austral, or Sonoran 



Lower Austral, or Lower Sonoran 



Upper Austral, or Upper Sonoran 



3. Transition 



4. Canadian 



5. Hudsonian 



6. Arctic-Alpine 



All of the zones, except the Tropical and Lower Austral, or Lower 

 Sonoran, are found in Oregon in varying extent, the Upper Sonoran and 

 Transition including most of the land surface of the State. The areas are 

 shown in color on the map of the Life Zones of Oregon, prepared by 

 Vernon Bailey, formerly of the Bureau of Biological Survey, which is 

 placed in an envelope at the end of this publication. 



Since the Tropical Zone does not occur in the State, the Austral, or 

 Sonoran, is considered first. So far as north or south zones are concerned 

 the terms "Austral" and "Sonoran" are synonymous, representing as they 

 do the subtropical fauna and flora of Mexico and the southern United 

 States. There are such marked differences east and west, however, that 

 Austral is now generally used to designate the eastern, or more humid 

 part, of the zone; and Sonoran, the western, or arid section. The word 

 "Sonoran" is therefore used to designate the arid part of the subtropical 

 belt. This zone has been further divided into a lower, or southern, and 

 an upper, or northern, section, and these sections represent almost as 

 distinct differences as those found between zones. Only the Upper Sonoran 

 part of the zone is represented in Oregon. 



In "The Mammals and Life Zones of Oregon," North American Fauna 

 No. 55, of the United States Biological Survey, Vernon Bailey (193 6) 1 has 



1 Dates in parentheses following an author's name refer to Bibliography, p. 609. 



