Locality Studied The observations reported in this paper were made during the spring of 1934 on the Brewer blackbird population of a 15-acre tract of chaparral and forest 14 miles northwest of Reno, Nevada. The period of observation, extending from May 17 to June 16, was preceded by a four-day examination of the area in an effort to locate and map the positions and contents of all nests (Fig. 1). During this provisional checkup, 91 nests were discovered, 42 being in various stages of construction, while many of the remainder had been finished so recently that they contained no eggs. During the following weekMay 17 to 2415 more nests were found. Nest No. 107 was not located until the 27th. This figure of slightly more than 7 nests per acre indicated a heavy infestation for the region. The zonation of the area is of interest because of its truly transitional aspect, lying as it does at the junction (as nearly as zonal junctures can be postulated) of Transition and Upper Sonoran life zones, on the lower, northwestern slopes of Peavine Mountain (Fig. 2). Southeast of the area studied, the slopes and ridges ascend to Peavine Peak, 8,270 feet high, while directly north lies Dry Lake, a symmetrical playa occupying the lowest point of adjacent Lawton Valley, at an elevation of 5,000 feet. The blackbird plot discussed here lies at an elevation of 5,500 feet, and is dominated by Transition floral elements, most prominent of which are Mountain Mahogany (Cercocarpus ledifolius), Jeffrey Pine (Pinus ponderosa je/reyi), White Fir (Abies concolor), and Incense Cedar (Libocedrus decurrens) (Fig. 3). Intrusive Upper Sonoran elements are represented by Sagebrush (Artemisia tridentata), Wild Peach (Prunus andersoni), Buckbrush (Purshia tridentata), and Serviceberry (Amelanchier alnifolia), the latter being characteristic of the Transition-Upper Sonoran zonal juncture. Buckbrush, an inhabitant of the extreme upper limits of the Upper Sonoran, is a consistent indicator of the proximity of the Transition, and in this region fringes the Jeffrey pine forest. Mountain Mahogany extends considerable distances from the pine stands, and is regarded, in this study, as the Transition zone index species, for it makes a sharper unit contact with the Upper Sonoran than any other Transition element. Initial observations began on May 13, at which time the nesting season was well under way, and continued for a month, during which time all surviving eggs had hatched, and the birds matured enough to leave the nest. In order to tabulate results more conveniently, check data, and formulate conclusions, the tract of 15 acres was divided into four divisions, East, West, Coyote Spring, and Meadow, lying, respectively, east, west, south, and north of the centrally-located cabin which the author occupied during the study. 417