The Influences of the Southwestern Deserts upon the Avifauna of California

Abstract

THE following paper is an outline of a rather intricate problem which really merits the most intensive investigation. This I at one time hoped to undertake, but its consummation seems to be indefinitely postponed at present. In addition, the very nature of the problem and the character of the area involved inhibit for many years a painstaking and comprehensive treatment of the subject presented, because of lack of information relative to the precise movements of so many Californian birds during the migrations. For this reason-that the data on which some of my conclusions are based are admittedly sketchy-these conclusions are advanced somewhat tentatively and subject to later revision in some cases, but the majority seem to be fundamentally sound. The southwestern deserts as here considered consist of those intensely arid areas of the Lower Sonoran Zone that, in Arizona, are situated west of the Babiquivari Mountains and south of the extensive Upper Sonoran stretches of Yavapai County; and in California, are situated east of the southern extensions of the Sierra Nevada and south of the jumble of mountain systems in the neighborhood of Death Valley. As a whole, this area is the most forbidding region in the United States-the most perfect desert barrier, theoretically, that we have. For vast stretches it supports only the scantiest desert bushes, chiefly creosote. Roughly, the Californian portion of this area may be divided into two sections. The northernmost, here termed the central desert, is located to the north of the San Bernardino and Chocolate ranges, and includes the Mojave Desert. The parts of this that are located away from the Colorado River, or in other words, the somewhat more elevated portions, are particularly unattractive in appearance, with vegetation seldom vigorous even in the lower washes. On the whole, summer temperatures are high but not excessive, save in certain localities, and winter ones are often decidedly frosty. The climate is chiefly influenced by the high cordillera on the west, and probably in some degree, by the mountain ranges on the north. The southern or Colorado Desert