Distribution: Two subspecies of Brewer's sparrow, Spizella breweri, are recognized, but only Spizella breweri breweri breeds in Utah; the more northerly subspecies - the timberline sparrow (S. b. taverneri) - is restricted to high elevation sites in Canada and Alaska. The Brewer's sparrow (S. b. breweri) is primarily a Great Basin species, but it occurs in shrub steppe habitats in all western states; it breeds throughout Utah and Nevada as well as in Montana, Wyoming, western Colorado, northern New Mexico, northern Arizona, eastern California, eastern Oregon, eastern Washington, and southern Idaho. Its range also extends to southwestern Saskatchewan and southeastern Alberta (Rotenberry et al. 1999). In Utah, Brewer's sparrows are common (Behle et al. 1985) to very common (Hayward et al. 1976) summer residents, breeding throughout the state in appropriate habitats. Densities in Utah are high in the northern and western parts of the state and highest in Rich and Summit counties (Sauer et al. 1997). Brewer's sparrows winter in southeastern California, southern Arizona, and southern New Mexico, south into Baja and the central states of Mexico (Rotenberry et al. 1999); they occur rarely in Utah during the winter (Behle et al. 1985), most often in the southwestern corner of the state (Sauer et al. 1997).
Ecology: Brewer's sparrows are considered Neotropical migrants, though some populations may travel only a short distance between breeding and wintering ranges. Northern populations migrate farthest south (Rotenberry et al. 1999) and Utah Brewer's sparrows probably winter in the Sonoran and Chihuahuan deserts of the southern United States and northern Mexico in mixed-species flocks with other sparrows. Brewer's sparrows typically arrive in Utah in mid-April and depart in mid-October (Behle and Perry 1975).
Upon arrival at breeding grounds, male Brewer's sparrows establish territories (usually about 0.5 ha [1.2 ac]) which are vigorously defended both vocally and physically (Reynolds 1981). Females arrive a few days after males, and although no noticeable courtship behaviors are exhibited, pairs form a few days later. Territory establishment and pair formation might be delayed by cold weather (Peterson and Best 1985).
Nests, tight cups of grass and forbs lined with finer materials such as hair, are constructed in mid-May. Nest construction is primarily by the female and takes about 5 days. Nests are typically placed between 20 and 50 cm (8-20 in) high; they are usually in the top half of the shrub (Peterson and Best 1985). Nests are usually located in patches of sagebrush that are taller and denser, with more bare ground and less herbaceous cover, than the surrounding habitat. In Idaho, nest shrubs averaged 69 cm (27 in) in height, whereas height of the surrounding habitat was 43 cm (17 in) (Peterson and Best 1985). The vast majority of Brewer's sparrow nests are in sagebrush; however, other shrubs are occasionally used (Rotenberry et al. 1999).
Clutch size is usually 3-4 eggs, occasionally 2 and rarely 5, and the clutch is layed at a rate of 1 egg per day. Incubation begins when the second-to-last egg is laid and lasts for 10-12 days (Rotenberry and Wens 1991). The female performs most of the incubation duties, but the male frequently remains near the nest and occasionally incubates and delivers food to the female. Brewer's sparrows will renest in a few days if the initial clutch is lost. Hatching begins in late May and peaks in the first 2-3 weeks in June (Howe et al. 1996). Hatching of altricial young takes place over a 1 to 2 days (up to 4 days) period and usually occurs in the mornings (Rotenberry et al. 1999). Both parents brood nestlings for 8 - 9 days. Adults feed nestlings almost exclusively insects (Petersen and Best 1986, Howe et al. in press) which are caught within 50 m of the nests (Rotenberry et al. 1999). Food items are delivered to the nestlings on average every 8 - 13 minutes (Howe et al. in press) and feeding frequency increases with age of the nestlings (Rotenberry et al. 1999). Nestlings remain in the nest area (less than 10 m [33 ft] from the nest) for several days before they are able to fly. Parents attend the nestlings after they leave the nest for several days, though it is not know exactly how long parents remain with the fledglings. Late nests (late June to mid July) may represent renesting after failed attempts or double brooding. Brewer's sparrows are frequent brown-headed cowbird hosts and often abandon parasitized nests (Rotenberry et al. 1999).
Brewer's sparrows are primarily insectivorous during the breeding season though their diet consists mostly of grass and weed seeds in winter. They glean insects from shrub foliage and bark and take seeds from the ground. The diet of Brewer's sparrow changes throughout the breeding season and differs between years (probably in relation to food availability). They feed on a wide variety of arthropods including spiders, leaf bugs, cicadas, snout beetles, caterpillars, crane flies, ants, and grasshoppers (Rotenberry et al. 1999). Nestlings are fed a similar diet, with most of the diet made up of caterpillars, butterflies, spiders, beetle larvae, crane flies, cicadas, and grasshoppers (Petersen and Best 1986, Howe et al. in press).
Brewer's sparrows breed primarily in shrub steppe habitats in Utah and are considered to be shrub steppe obligates by Braun et al. (1976). However, Brewer's sparrows may also be found in high desert scrub (greasewood) habitats, particularly where these habitats are adjacent to shrub steppe. They may also breed in large sagebrush openings in pinyon-juniper habitat or coniferous forests.