Both body size and coloration vary slightly to moderately across the range of this species. Wing and tail length vary little but generally increase from north to south, although this “trend” reverses south of the Isthmus of Tehuantepec in s. Mexico, such that the smallest birds are found in coastal Nicaragua (Webster 1973). Bill length is greatest in s. Florida and perhaps elsewhere in the se. United States (Phillips 1991). In both sexes, reddish brown on the ventrum generally is darkest and richest (i.e., the most red or chestnut) and contrasts most sharply with the white belly in e. North America and is paler and more cinnamon, with reduced contrast, to the west (e.g., Arizona) and south (e.g., Middle America). Coloration is clinal but falls into roughly four groups: e. North America (including ne. Mexico), Central Plateau of Mexico (north to Arizona), highlands south of Isthmus of Tehuantepec, and lowland pine savanna of Nicaragua and Honduras. The hue of brown dorsally on females is clinal north to south, being grayest in the north, browner in Mexico, and reddest in Central America, excepting birds in lowland ne. Nicaragua, which are paler with a more yellowish hue. Females in s. Mexico and Central America have the dorsum more extensively blue.
Seven subspecies, following Phillips (1991), who updated classifications by Ridgway (1907), Ripley (1964), and Webster (1973). Characters used to diagnose taxa are based chiefly on coloration, which requires comparison of specimens in fresh plumage; this species' lack of a Prealternate molt yields worn birds in spring and summer, which confounds assessments. Some subspecies seem weakly differentiated; some work is needed, using fresh material and large sample sizes.
S. s. sialis (Linnaeus, 1758). Includes Muscicapa aurea Vieillot, 1818; M. azurea Stephens, 1826; S. wilsonii Swainson, 1827; S. arctica Nelson, 1879; S. s. grata Bangs, 1898; and S. s. episcopus Oberholser, 1917. Breeds in e. North America, east of the Rocky Mts., from s. Canada south to e.-central Coahuila and through Florida [type locality = South Carolina]; winters southward within range and to Bermuda, n. Mexico, Cuba; casual to Arizona. Ventrum dull and dark rufous to chestnut, the breast contrasting sharply with the white belly; male dorsum dark purplish blue; chestnut edging to mantle feathers lacking; body size averages small (male wing < 104 mm).
S. s. bermudensis (Verrill, 1901). Resident on Bermuda. Like S. s. sialis, but ventrum darker and deeper red and dorsum of males brighter blue lacking purplish tones (Avery et al. 2014).
S. s. nidificans (Phillips, 1991). Resident in mountains from sw. Tamaulipas to central Veracruz [type locality = 18 km east of Ciudad Maiz, San Luis Potosí]. Like S. s. sialis, but more richly colored overall, ventrum darker, and mantle feathers of male edged in chestnut (when fresh); averages large (male wing > 104 mm).
S. s. fulva (Brewster, 1885). Resident from s.-central and se. Arizona south through pine and pine–oak habitats of w. Mexico south to the Isthmus of Tehuantepec [type locality = Santa Rita Mts., Arizona]. Similar to S. s. sialis, but ventrum paler and duller cinnamon (with less contrast to white belly) and mantle browner (less gray); female browner.
S. s. guatemalae (Ridgway, 1882). Supercedes S. albiventris Baird, 1858, a nomen nudum. Resident in mountains of Chiapas and adjacent Guatemala [type locality = Guatemala]. Similar to S. s. nidificans, but duller, paler, and less reddish.
S. s. meridionalis (Dickey and van Rossem, 1930). Resident in highlands of central and w. Honduras, n. El Salvador, and w. Nicaragua, and probably w. Belize [type locality = Los Esesmiles, Chalatenango, El Salvador]. Like S. s. guatemalae, but more brightly colored, ventrum redder, and smaller (male wing < 99–103 mm); female dorsum bluer.
S. s. caribaea (Howell, 1965). Resident in lowland pine savannah of the Mosquito Coast of e. Honduras and ne. Nicaragua [type locality = [6 km NW of Leicus Creek, Comarca de El Cabo, Nicaragua]. Similar to S. s. meridionalis, but ventrum paler and small (male wing < 97 mm; see Howell 1965).
The family Turdidae (thrushes), which occurs on every continent save for Antarctica, is characterized by having the juvenile plumage spotted (usually), a single annual molt, and undivided (booted) tarsus (except lower portion) rather than a scutellate horny covering over the tarsus. The family is a component of the superfamily Muscicapoidea (Ericson and Johansson 2003, Ericson et al. 2003) along with the Mimidae (thrashers), Sturnidae (starlings), Cinclidae (dippers), and Muscicapidae (Old World flycatchers).
Protein electrophoresis (Avise et al. 1980) and DNA–DNA hybridization (Sibley and Ahlquist 1990) implied that the genus Sialia, the three species of bluebirds from North America, is quite distinct from turdine thrushes. Instead, Sialia is sister to Myadestes, the solitaires of North and South America and Hawaii (Klicka et al. 2005). Within the genus, on the basis of coloration it has long been postulated that S. sialis is sister to S. mexicana, the Western Bluebird. Sequence divergence of a portion of mitochondrial DNA is on the order of 5.0% ± 0.67 SD, consistent with a late-Pliocene divergence of these species dating to 2.45 mya, if the "mitochondrial clock" is calibrated properly (Klicka and Zink 1997).
Plumage similarities aside, a more exhaustive molecular analysis suggested that S. sialis is sister to S. currocoides, the Mountain Bluebird (Klicka et al. 2005). These putative sister species do form mixed pairs and hybridize where ranges meet in the n. Great Plains states and s. Prairie Provinces: records across 21 years in Manitoba and Saskatchewan include most combinations of S. sialis, S. currucoides, and backcrosses involving hybrids, although hybridization is rare in the wild, involving only 0.1–0.6% of sympatric breeding pairs (Rounds and Munro 1982, 1983).