THE AREA CONSIDERED

The region studied is designated in this report as the 'Yosemite region,' or, more precisely, as the 'Yosemite section.' It involves, as shown on the accompanying map (pl. 62), a narrow rectangular area, 89-1/4 miles in length by 17-1/3 miles in width. It reaches from the eastern margin of the San Joaquin Valley eastward across the mountains to include the western margin of the Great Basin, around Mono Lake, and thus constitutes a typical cross-section of the central Sierra Nevada. The altitudes range from 250 feet, at Snelling, to slightly over 13,000 feet, on Mount Lyell. The total 'map' area is 1547 square miles. Yosemite Valley is included in its entirety; the Valley ends of the Wawona and Big Oak Flat roads are within the 'section,' as are the greater parts of the Coulterville and Tioga roads. But neither the Mariposa Grove of Big Trees nor Hetch Hetchy Valley is included.

Within the limits of this 'section,' the members of our field party traveled over most of the regular trails (routes are shown on the map); in addition they sought out high points from which practically every square mile of territory could be seen and mapped as to life zone. All together, 40 collecting stations were occupied by different members of our party. The number of persons working at any one station at one time varied from one to five. Certain camps such as those in Yosemite Valley, at Porcupine Flat, on Tuolumne Meadows, and at the Farrington ranch were 'base camps,' from which short trips were taken in different directions. At all the places marked on the map as collecting stations, trapping for mammals was done on one or more nights.

STATISTICS OF FIELD WORK

The first regular field work of the Yosemite Survey was a reconnaissance trip by the senior author in the autumn of 1914. (Both authors were already familiar with the lay of the land from previous visits to Yosemite Valley and its environs.) Formal field work was instituted on November 19, 1914, and continued until January 9, 1915; it was commenced again on May 15, 1915, and continued until July 31; it was again taken up on August 16 and carried on until November 23 (1915). In 1916 continuous work was carried on in the neighborhood of Mono Lake from April 26 until July 6. That same year, two brief trips were made into Yosemite Valley, at the end of February and at the end of April. In 1919 work in the western part of the region was carried on from May 5 to 27; and in 1920 work was in progress there from June 20 until August 11.

Nine hundred and fifty-seven 'man-days' (one man in the field one day) were put in. The field notes written occupy 2001 pages, and the specimens secured by our regular field men number 4354. The photographs obtained number 700. In addition as indicated elsewhere, much valuable information and many important specimens were secured from residents in the Yosemite region.

All the materials upon which this report is based, including specimens, maps, notebooks, and photographs, are now contained in the Museum of Vertebrate Zoology of the University of California and are the property of the State of California.

FIELD PERSONNEL

Eight different persons participated at one time or another in the field work of the Yosemite Survey; 248 days were put in by Joseph Grinnell, 170 days by Tracy I. Storer, 111 days by Walter P. Taylor, 110 days by Joseph Dixon, 103 days by Charles L. Camp, 92 days by Gordon F. Ferris, 91 days by Charles D. Holliger, and 32 days by Donald D. McLean. It should be understood that whatever degree of accuracy and fullness the present report may possess rests upon the diligence, as field collectors and observers, of each and every one of these persons.

FIELD METHODS

The general plan of work was much the same at all the collecting stations. It was of course essential, in the interests of truth and scientific accuracy, that many specimens be obtained in order that correct identification of the species might be insured. Hence, each member of the party kept out a line of mouse and rat traps for the capture of the various species of small mammals. These were set in 'likely' places: along stream banks for shrews; in runways of meadow mice; about brush heaps or downed logs for white-footed mice, and so on. Special traps were set for moles, for pocket gophers, and for carnivorous species. These traps were baited the last thing each evening and were visited early the next morning so as to collect the animals caught before they might be harmed by sunshine or by insects. Where chipmunks abounded, or ground dwelling birds were numerous, traps were often visited during the day to recover such animals as were caught; or else the traps were purposely sprung in the morning and reset again in the evening in order to avoid capturing mammals or birds not needed for specimens. Birds were obtained, when necessary, by shooting selected individuals. Many reptiles were captured by hand, although some of the swifter ones could be obtained only by shooting.

But the taking of specimens was only one of several lines of activity. The morning of each day was usually spent away from camp observing and making notes upon the various species to be seen—their local distribution, forage habits, nesting behavior, and all the other observable features connected with their life histories. Each member of the party carried a notebook (journal) in which the observations of each day were recorded. Notes on the behavior of individual animals were written down usually while observation was in progress, to insure the entry of details with accuracy. When nests, burrows, or other 'workings' were examined, the measurements and diagrams were entered directly in the journal. Censuses were gathered as they were taken, the individuals pencil-checked one by one according to the method described fully elsewhere (p. 22). Photographs were taken of 'associations,' workings, tracks, and nests, and these materially supplemented the written data.

LIMITATION OF TIME

It became necessary, as in all such undertakings as this, arbitrarily to fix upon a date beyond which no further matter would be incorporated into this report. This date was set as December 31, 1920. Even though important new facts have been reported from the Yosemite region by competent observers since that date, we have forborne inclusion thereof. Inevitably, such additions will continue to be made so long as people with an interest in natural history visit the Yosemite region. The natural history resources will never become exhausted; and that is one fascinating feature of this field of inquiry. Our efforts, then, have been to assemble all the available information concerning the vertebrate animals of the Yosemite region up to and including December, 1920.