Nest and Nest-Site Characteristics of a Western Population of Fox Sparrow (Passerella iliaca)

Abstract

Relatively little is known about nesting and breeding habits of the fox sparrow due to its secretive nature on the breeding grounds (Linsdale, 1928; Austin, 1968; Threlfall and Blacquiere, 1982). Accounts of fox sparrow nests describe materials used, species of plant the nest was found in or under, and general characteristics of the habitat in the area, but do not give nest measurements or describe vegetation immediately surrounding the nest-site (Austin, 1968; Threlfall and Blacquiere, 1982). Bendire (1889) and Pierce (1921) each gave measurements of a single representative nest for some subspecies, while Mailliard (1921) reported the ranges of measurements of 14 nests from a single population at Lake Tahoe, California. Herein we provide measurements of 23 fox sparrow nests as well as descriptions of physical and vegetative characteristics associated with 25 nest-sites from a single population. These data, viewed as phenotypic extensions with possible effects on fitness (as argued in Dawkins, 1982; Lent, 1992), add to the growing body of information about individual and geographic variation in this species (Linsdale, 1928; Swarth, 1920; Zink, 1986, in press). Fox sparrows were studied from late May to early July 1989 in Sequoia National Forest, 7.5 km west of Hume, Fresno County, California (36048'N, 118059'W, altitude 1,950 m). The study site is at the boundary between two fox sparrow subspecies (P. i. stephensi and P. i. megarhyncha) interconnected by clinal variation (Zink, 1986); therefore, we do not assign a subspecies to the population we studied. Habitat at the study site consists of mostly chaparral (the most abundant species are listed in Table 1) and also some mixed coniferous forest including giant sequoia (Sequoiadendron giganteum), white fir (Abies concolor), ponderosa pine (Pinus ponderosa), and sugar pine (Pinus lambertiana). Nests were located by observing adults and by walking through patches of vegetation while attempting to flush incubating or brooding adults. Nest and nest-site characteristics were measured