Spizella breweri

The taxonomic status of the two subspecies S. b. breweri and S. b. taverneri is controversial. Sibley and Monroe (1990) cited personal communications from J. C. Barlow and W. B. McGillivray in listing S. taverneri as a distinct species (breeding in the subalpine shrublands of western Canada), based on differences in vocalizations, morphology, and ecology (see also AOU 1998, Rotenberry et al. 1999). Klicka et al. (1999) also concluded that the two taxa should be regarded as separate species; this conclusion, however, was disputed by Mayr and Johnson (2001).

See Zink and Dittmann (1993) for a hypothesis for evolution in the genus Spizella. See Dodge et al. (1995) for a comparison of phylogenies derived from two molecular data sets for the genus Spizella; among other results, monophyly of Spizella including the American Tree Sparrow was supported.

Fairly large range in western North America; declining in many areas of the U.S.

BREEDING: subspecies BREWERI: southern British Columbia, southern Alberta, southwestern Saskatchewan, Montana, and southwestern North Dakota, south to southern California (northern Mojave Desert), southern Nevada, central Arizona, northwestern New Mexico, central Colorado, southwestern Kansas, northwestern Nebraska, and southwestern South Dakota (AOU 1998, Rotenberry et al. 1999). Mapped BBS data show centers of summer abundance in the Great Basin and Wyoming Basin (Sauer et al. 1997). Subspecies TAVERNERI: southwest Alberta, northwest British Columbia, southwest Yukon, and southeast Alaska (Rotenberry et al. 1999). NON-BREEDING: southern California, southern Nevada, western and central Arizona, southern New Mexico, and west Texas, south to southern Baja California, Sonora, and in highlands from Chihuahua, Coahuila, and Nuevo Leon south to northern Jalisco and Guanajuato (Terres 1980, AOU 1998, Rotenberry et al. 1999). Highest wintering abundance in Arizona (21.41 birds per 100 survey hours) 1959-1988 (Sauer et al. 1996).

Number of occurrences not known but relatively widespread.

Direct cause of widespread decline on breeding grounds is uncertain, but possibly linked to widespread degradation of sagebrush habitats.

HABITAT LOSS, FRAGMENTATION: A shrub obligate that is threatened by large scale reduction and fragmentation of sagebrush habitats occurring due to a number of activities, including land conversion to tilled agriculture, urban and suburban development, and road and power-line rights of way. Range improvement programs remove sagebrush by burning, herbicide application, and mechanical treatment, replacing sagebrush with annual grassland to promote forage for livestock.

GRAZING: Grazing can trigger a cascade of ecological changes, the most dramatic where invasion of non-native grasses escalates the fire cycle and converts sagebrush shrublands to annual grasslands. Historical heavy livestock grazing altered much of the sagebrush range, changing plant composition and densities. West (1988, 1996) estimates less than 1 percent of sagebrush steppe habitats remain untouched by livestock; 20 percent is lightly grazed, 30 percent moderately grazed with native understory remaining, and 30 percent heavily grazed with understory replaced by invasive annuals. Effects of grazing in sagebrush habitats complex depending on intensity, season, duration and extent of alteration to native vegetation.

INVASIVE GRASSES: Cheatgrass readily invades disturbed sites, and has come to dominate the grass-forb community of more than half the sagebrush region in the West, replacing native bunchgrasses (Rich 1996). Crested wheatgrass and other non-native annuals have also fundamentally altered the grass-forb community in many areas of sagebrush shrub-steppe, altering shrubland habitats.

FIRE: Cheatgrass has altered the natural fire regime in the western range, increasing the frequency, intensity, and size of range fires. Fire kills sagebrush and where non-native grasses dominate, the landscape can be converted to annual grassland as the fire cycle escalates, removing preferred habitat (Paige and Ritter 1998).

BROOD PARASITISM: An occasional host for brown-headed cowbird (MOLOTHRUS ATER). Prior to European-American settlement, were probably largely isolated from cowbird parasitism, but are now vulnerable as cowbird populations increase throughout the West and where the presence of livestock and pastures, land conversion to agriculture, and fragmentation of shrublands creates a contact zone between the species (Rich 1978, Rothstein 1994). Frequency of parasitism varies geographically: 13 percent in Idaho, 5 percent in central Oregon, 0 percent in northern Nevada, 52 percent in southeastern Alberta. Extent of impact on productivity unknown (Rotenberry et al. 1999). Usually abandoned parasitized nests and cowbird productivity was lower than Brewer's (Biermann et al. 1987). Rich (1978) also observed cowbird parasitism on two nests in Idaho, both of which were abandoned.

PREDATORS: Intense episodic predation by Townsend's ground squirrel (SPERMOHPILUS TOWNSENDII). Other documented or suspected nest predators (of eggs and nestlings) include: gopher snake (PITUOPHIS MELANOLEUCUS), loggerhead shrike (LANIUS LUDOVICIANUS), common raven (CORVUS CORAX), black-billed magpie (PICA PICA), long-tailed weasel (MUSTELA FRENATA), least chipmunk (TAMIAS MINIMUS), western rattlesnake (CROTALUS VIRIDIS), and other snake species. Nest predation significant cause of nest failure; impacts vary geographically and temporally. In 1976-1977, predation rate ranged from 11 percent in Oregon to 86 percent in Idaho and 100 percent in Nevada. Predation ranged from 0 to 37 percent in Oregon, 1976-1980.

American kestrel (FALCO SPARVERIUS), prairie falcon (FALCO MEXICANUS), loggerhead shrike, coachwhip (MASTICOPHIS FLAGELLUM) reported preying on adults (Rotenberry et al. 1999). Significant negative correlation between loggerhead shrike and Brewer's sparrow density observed (Wiens and Rotenberry 1981). Levels of predation during non-breeding season have not been reported.

PESTICIDES: Potentially affected by insect or weed control programs. Nest success not affected by applying 2,4-D Herbicide on big sagebrush plants with nests. Bird densities on treated area, however, were 67 percent lower one year after application and 99 percent lower two years after application (Schroeder and Sturges 1975).