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Habitats. —Birds have adapted themselves to many different environments, usually called habitats. This helps alleviate the competition between different species for food and nesting sites. To cite a few examples—ducks feed in open water and nest in marshes, swifts feed in the air and nest on the cliffs, kingfishers dive for fish in streams and nest in the banks, sandpipers feed on the shores and nest on the ground, woodpeckers frequent forests and feed on wood-inhabiting insects and nest in tree trunks, kinglets feed and 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 Yosemite region is reflected in the richness of its birdlife. 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 altitude 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, Hudsonian, 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, coming to the park from the Great Valley of California, starts his journey in the Lower Sonoran life zone and can go by automobile to Tioga Pass in the Hudsonian zone having passed through the Upper Sonoran, Transition, and Canadian life zones enroute. By walking 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 summits 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, are 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, however, that the lines between zones are seldom sharp and that local effects of slope exposure, air currents, precipitation, etc. may modify the local temperature 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. Keeping 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 Arctic-alpine zone of the Sierran crest, an altitudinal range 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.
LIFE ZONES OF THE YOSEMITE REGION
LOWER SONORAN — Sea level to 500 feet. Grassland with scattered valley oaks; otherwise largely treeless except along streams. Confined to the lowlands of the Great Valley Birds—Red-tailed hawk, barn owl, burrowing owl, common nighthawk, horned lark, brown-headed cowbird, American goldfinch, blue grosbeak, least vireo, yellowthroat, mockingbird.
UPPER SONORAN — 500 to 3,000-4,000 feet. Digger pine, blue oak, interior live oak, scrub oak, mountain mahogany, toyon and other chaparral plants. The brushland or chaparral zone of the foothills. Birds—Nuttall’s woodpecker, scrub jay, sage sparrow, brown towhee, Hutton’s vireo, California thrasher, Bewick’s wren, plain titmouse, wrentit, western bluebird.
TRANSITION — 3,000-4,000 to 6,000-7,000 feet. Ponderosa pine, sugar pine, incense-cedar, white fir, Douglas-fir, black oak, canyon live oak, bigleaf maple, dogwood. Birds— Pygmy owl, band-tailed pigeon, Caifornia purple finch, solitary vireo, Nashville warbler, black-throated gray warbler, Macgillivray’s warbler, winter wren.
CANADIAN — 6,000-7,000 to 8,000-9,000 feet. Red fir, Jeffrey pine, lodgepole pine, white pine, quaking aspen, chinquapin. Birds—Blue grouse, goshawk, calliope hummingbird, Williamson’s sapsucker, Hammond’s flycatcher, Cassin’s purple finch, Townsend’s solitaire, Lincoln’s sparrow
HUDSONIAN — 8,000-9,000 to 11,000-11,500 feet. Whitebark pine, mountain hemlock. Birds—Black-backed three-toed woodpecker, mountain bluebird, pine grosbeak, Clark’s nutcracker, white-crowned sparrow.
ARCTIC-ALPINE—11,000-11,500 feet and above. Dwarf willow, various dwarf or matted flowering plants, and turf-forming grasses and sedges—the area above timberline. Birds—Gray-crowned rosy finch.
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