“When we come upon the Brewer Sparrow, we are ready to wager that here the dame [Nature] has done her utmost to produce a bird of non-committal appearance. Mere brown might have been conspicuous by default, but brownish, broken up by hazy streakings of other brownish or dusky—call it what you will—has given us a bird which, so far as plumage is concerned, may be said to have no mark of distinction whatever—just bird.”
William Leon Dawson ( Dawson 1923 : 313)
Although referred to as nondescript, drab, or even “non-committal,” we prefer “subtle” to describe the appearance of Brewer's Sparrow. We agree with Dawson that this bird of undistinguished plumage harmonizes well with the muted colors of its major habitat, sagebrush shrublands. Much of the arid landscape of the intermountain West is dominated by a sea of sagebrush, occasionally mixed with other shrubs and grasses, and the Brewer's Sparrow is by far the most abundant bird there during spring and summer. And although its plumage may be drab, its song certainly is not; a spring morning in the Artemisia shrub-steppe vibrates with the buzzy trills, ascending and descending, of dozens of males proclaiming their territory and attempting to attract a mate.
Typically, pairs form shortly after arrival on the breeding grounds in spring, building a small open-cup nest in sagebrush and laying 3 eggs. If not discovered by a predator during the 20–22 days it takes from egg-laying to fledging, 3 chicks are produced. Time permitting, the pair attempts a second brood. By late summer, territories have broken down and birds begin moving about in family groups and small flocks; by early fall, southward migration has begun. Throughout the year, Brewer's Sparrows remain in shrublands, migrating through the southern Great Basin and foothills of the Rockies and the Sierra Nevada, and wintering in the desert scrub of the southwestern United States and northern Mexico, often in the company of other sparrow species. Although its bill morphology is typical for a sparrow, much of this species' diet consists of arthropods. As befits an aridland's species, its water economy is excellent, and it can exist for long periods without drinking.
This sparrow was named by John Cassin in 1856 in honor of his friend, Thomas M. Brewer, a Bostonian originally educated as a physician. Brewer (1814–1880) was a prominent ornithologist of the mid-1800s, noted particularly for his contributions to the study of birds' eggs. Cassin (1813–1869) named almost 200 species of birds, most during his tenure at the Academy of Natural Sciences in Philadelphia. He separated the Brewer's Sparrow from the similar Clay-colored Sparrow (Spizella pallida) from specimens in the Academy's holdings, which included many skins collected during the numerous military expeditions that explored the American West during the nineteenth century.
Recent (1980s and 1990s) surveys have shown breeding numbers of Brewer's Sparrows to be in significant decline throughout the species' range. As for many declining species, the causes are uncertain, but they may be related to fundamental changes in shrubland ecosystems being brought about by agriculture, grazing, and the invasion of exotic plant species. And although the species can be abundant over large landscapes, it inhabits an area relatively sparsely populated by humans (and their universities, field stations, and other movers of research); as a result, comparatively little is known about major aspects of its biology. Much of the information we do have is a result of several studies on the population and community ecology of shrub-steppe passerines in the 1970s and 1980s conducted in eastern Oregon (J. Rotenberry, J. Wiens, and colleagues on habitat associations, population biology, and behavior) and southeastern Idaho (L. Best, K. Petersen, T. Reynolds, and T. Rich on reproductive biology, habitat associations, and especially nest and nesting biology). By bringing together everything known, we hope to be able to understand and potentially reverse this decline.