LIFE ZONES xxi 



C. Species belonging to one or the other slopes but penetrating 

 across, illustrated by the Cactus Woodpecker, California Thrasher, 

 and Valley Quail. 



D. Species represented in each of the two areas but by obviously 

 closely related forms. (Grinnell and Swarth.) 



The precipitous eastern slope of San Jacinto begins at 500 feet 

 elevation in the Lower Sonoran of the Colorado Desert, the hot 

 winds of which carry that Zone from below sea-level to 4000 feet. 

 Upper Sonoran, extending entirely around the mountain, on the 

 desert slope reaches 1500 feet higher than on the gentler western 

 slope, where ocean breezes and moisture have their influence. 

 Greasewood is the most general "Zone indicator" and the Wren- 

 tit is the omnipresent bird of the chaparral. Transition Zone on 

 this desert slope is contracted into a belt of hardly more than 

 1500 feet in vertical width, as compared with more than 3000 

 feet on the western side. White-headed Woodpeckers, Audubon 

 Warblers, and Western Tanagers nest in this forest belt. Yellow 

 pine and black oak characterize the lower, and white fir dominates 

 the upper portion of the Transition. For purposes of bird distri- 

 bution, the Boreal Region may be considered as one division. The 

 Arctic-Alpine Zone occupies a small patch on the very summit of 

 the mountain. Extending down all sides to altitudes of 8500-9000 

 feet are the Canadian and Hudsonian Zones, which, taken to- 

 gether, spread out from the main peak over all the higher parts of 

 the mountain. The Murray (lodgepole) and limber pines are the 

 only trees inhabiting the higher slopes and ridges and are restricted 

 to these two Zones. (Hall.) 



It is desirable that the student become intimately acquainted 

 with the Life Zones of a locality, noting first the plant " indicators " 

 as being the simpler and more obvious forms, and then the 

 characteristic birds of the various Zones. He will then have vivid 

 personal experiences which will give meaning and usefulness to 

 the terms Sonoran, Transition, and Canadian. There is probably 

 no more striking exemplification of Life Zones on the continent 

 than is to be found in the Southwestern United States, and no 

 other one subject contributes more to an understanding of birds 

 and plants than does the study of Life Zones. 



