76 YOSEMITE NATURE NOTES 



nest among the boughs of coniferous trees, and rock wrens feed and nest 

 among the rocks. Knowing the habitats frequented by birds often helps 

 to corroborate an identification, for you learn to look for certain species of 

 birds in particular habitats. 



Life-zones. — The variety of living conditions available to birds in the 

 Yoseniitc region is reflected in the richness of its bird life. In an airline 

 distance of 70 miles, between the western edge of the foothills and the 

 Sierran* crest, changes in kinds of plants and animals occur that are similar 

 to those found over the vast area between southern United States and the 

 Arctic tundra, a distance of about 2,000 miles. With changes in either alti- 

 tude or latitude, from regions of high to those of low temperature, the 

 character of the native plant and animal life varies. For convenience in 

 biological study, characteristic forms of life have been grouped into belts 

 called life-zones, in accordance with changes in climate. In California, 

 made possible by its varied topography and climate, there are six such 

 zones — the Lower Sonoran, Upper Sonoran, Transition, Canadian, Hud- 

 sonian, and Arctic-alpine. In this book the last three are sometimes jointly 

 referred to as the Boreal zone. The intermediate region, the Transition 

 life-zone, often contains considerable numbers of plants and animals from 

 the adjacent zones and is thus appropriately named. Except for this zone, 

 each zone has been named for a latitudinal region with a characteristic 

 type of vegetation and animal life. For example, the Canadian life-zone, 

 in a latitudinal sense, consists of a broad strip of coniferous forest with its 

 associated fauna, that extends across much of southern Canada. 



The visitor to Yosemite, entering the park from the Great Valley of 

 California, starts his journey in the Lower Sonoran life-zone and can go 

 bv automobile to Tioga Pass in the Hudsonian zone having passed through 

 the Upper Sonoran, Transition, and Canadian life-zones enroute. By walk- 

 ing up the slope of Mount Dana, less than 2 miles by trail from the pass, 

 he reaches the coldest zone, the Arctic-alpine, which encompasses the sum- 

 mits of the highest peaks in the park. There, above timberline is to be found 

 the dwarf willow, short dense grasses and sedges, lichens and other low 

 growth, like that of the Arctic tundra. Thus within an airline distance of 

 50-70 miles, one can traverse all 6 life-zones in the western United States. 



To aid in the search for birds and in developing an understanding of 

 their occurrence, the life-zone range is given for all species except those 

 frequenting Yosemite only casually. Determination of the zone depends 

 upon recognition of certain "indicator" species of plants and animals. 

 Trees, where present, arc the most conspicuous indicators of a zone. The 

 best "indicators" are confined to the zone in question, or in the case of 

 animals, are those chiefly resident (especially as breeding) within it and 

 which therefore serve as good landmarks. It must be kept in mind, how- 

 ever, that the lines between zones are seldom sharp and that local effects 

 of slope exposure, air currents, precipitation, etc. may modify the local tem- 

 perature and consequently the details of the zonal picture. Furthermore 

 animals, especially birds and certain mammals, that have good powers of 

 locomotion, and even those regarded as good indicators, may sometimes 

 transgress zonal boundaries. KeejDing in mind the foregoing reservations, we 

 may list indicators for the life-zones beginning with the Lower Sonoran 

 zone of the Great Valley and extending to the Sierran crest, an altitudi- 

 nal ranee from 200 to over 13.000 feet. Altitudes given are for the western 

 slope of the Sierra and are approximate. All zones tend to be higher on 

 the east side of the Sierra. 



