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Small thrush: total length 16–21 cm; mass 29–32 g for females, 28–31 g for males (ranges are means from multiple studies). Male has rich blue upperparts, contrasting with red orange throat, breast, and flanks and white lower belly and undertail-coverts. Chin white or red orange. Outer rectrices have narrow white border. Female has blue gray upperparts, with gray brown wash across back; suggestion of white eye-ring on some individuals; wings and tail washed with dull blue; underparts paler orange than on male; white border of outermost rectrix is broader than on male (
Dunn, J. (1981). The identification of female bluebirds. Birding 13:4-11.
Dunn 1981). Both sexes have short, stout black bill, slightly notched at tip. Birds in Basic I plumage are generally duller and more grayish; outermost primary-covert lacks blue, is tapered (not pointed), and has broad buff or whitish border (
Pitts, T. D. (1985a). Identification of second-year and after-second-year eastern bluebirds. J. Field Ornithol. 56:422-424.
Pitts 1985a). Juvenile plumage browner, with white streaking above and dusky spotting below, and white eye-ring; sexes identifiable—males having blue in wing and tail, wing coverts, and narrower buff border on outermost rectrix (<0.5 mm;
Pinkowski, B. C. (1974d). Criteria for sexing eastern bluebirds in juvenile plumage. Inland Bird Banding News 46:88-91.
Males relatively easily distinguished from other bluebirds, females less so. Western Bluebird most similar in shape and behavior, but Mountain Bluebird (Sialia currucoides) is more often encountered in the range of the Eastern Bluebird, the 2 hybridizing (rarely) in the n. Great Plains. Eastern Bluebird has shortest primary extension (difference between longest primary and longest secondary is 24–34 mm;
Pyle, P. (1997). Identification Guide to North American Birds Part I: Columbidae to Ploceidae. Slate Creek Press, Bolinas, CA, USA.
Pyle 1997c); Western, and especially Mountain, bluebirds have longer primary extension on folded wing, primaries of Mountain Bluebird extending nearly to tip of tail at rest. Western Bluebirds are darker blue above; throat blue or gray; nape often contrasts with brown of back (chestnut patch on back of Western Bluebirds varies with some individuals lacking the patch). Underparts of Eastern Bluebird are crisper, usually with a distinct contrast between the white of the lower belly and the redder color of the breast; Western Bluebirds have grayish lower belly, undertail-coverts, and throat. On female Eastern Bluebirds, throat color usually extends to the side of the neck, unlike on Western and Mountain bluebirds. In addition to having proportionately longer wings, Mountain Bluebirds also have longer bills and legs; in fresh plumage in fall, they have more prominent whitish edgings on wing feathers, but also more reddish on breast, although still grayer than Easterns. See
Dunn, J. (1981). The identification of female bluebirds. Birding 13:4-11.
Zimmer, K. J. (1985). The Western birdwatcher. Englewood Cliffs, NJ: Prentice-Hall.
Zimmer 1985, and
Kaufman, K. (1992c). The practiced eye: bluebirds. Am. Birds 46:159-162.
Kaufman 1992c for more details.
Eastern Bluebirds have 10 primaries (the outermost, p10, reduced in length), 9 secondaries (including 3 tertials), and 12 rectrices. Geographic variation in appearance slight to moderate. The following molt and plumage descriptions pertain to the widespread nominate eastern North American subspecies S. s. sialis; see Systematics: Geographic Variation for appearance variation in up to six other recognized subspecies in southwestern North America, Florida, the Caribbean region, and Mexico to Central America. No geographic or sex-specific variation in molt strategies reported, although variation in average timing and extent likely occurs with latitude of breeding, responding to variable environmental and migratory constraints, day-length regimes, and breeding seasonality.
Following based primarily on detailed plumage descriptions of Dwight (1900), Ridgway (1907), Oberholser (1974), and Clement (2000); see Pinkowski (1974c), Pitts (1985), Plissner et al. (1995), and Pyle (1997a) for age-related criteria. Sexes show different appearances in all plumages; definitive plumage assumed at Second Basic Plumage.
Present primarily Apr–Jun in North America, in the nest. Hatchlings naked except for sparse, dingy gray to dark drab down or prepennae feathers (single row), occurring on the capital tract, in scapular region of the humeral tract, and along the spinal tract (2 rows).
Juvenile (First Basic) Plumage
Present primarily Jun–Aug in North American populations. Upperparts dusky brown to fuscous, the back feathers and scapulars with white spots and/or streaks; wings and tail, when fresh, as in Definitive Basic Plumage but blue coloration duller or washed grayish (see below), upperwing coverts brownish with terminal whitish or buffy white spots and streaks, the greater coverts and tertials also edged buff; sides of head and neck olive-brown; underparts dull whitish with dusky spots and streaks. Juvenile body feathers (especially undertail coverts) filamentous due to lower barb density than feathers of later plumages.
Among juveniles Male and Female can be distinguished by 13 d old by plumage variation but not size (Pinkowski 1974c), prior to day 13 sex assignment error prone (PAG). In females the juvenile primaries, primary coverts, and secondaries are dull, dingy grayish with little blue and they are more extensively and brighter blue in males. The back of females is generally lighter gray and spots may average larger than in males. The rectrices of females are dingy blue on the basal half and dull distally and the outer rectrix (r6) has a distinct and prominent broad white margin greater than 0.5 mm; in males rectrices are blue nearly throughout their length and the margins to r6 are less than 0.5 mm wide.
"First Basic" or "Basic I" plumage of Humphrey and Parkes (1959) and later authors; see revision by Howell et al. (2003). Present primarily Sep–Aug in North American populations. Male and Female similar to Definitive Basic Plumage but can be separated by molt limits between worn juvenile and fresh formative upperwing and tail feathers (Pyle 1997a): 3–10 inner greater coverts replaced, bluish in females and bright blue in males, contrasting with duller and grayer retained outer coverts; 1–3 tertials often replaced, contrasting with older retained juvenile tertials and secondaries; primary coverts duller and grayer, with reduced blue coloration by sex, contrasting with newer formative greater coverts, the outermost primary covert rounded and brownish (female) or dull bluish (male) with broad buff fringes (Pyle 1997a, Fig. 231); retained juvenile outer primaries and rectrices (if present) thinner, more pointed, browner, and relatively more worn (ca. 29% of birds show molt limits between juvenile and formative rectrices and ca. 31% show entirely replaced rectrices, resembling those of Definitive Basic Plumage).
Definitive Basic Plumage
Present primarily Sep–Aug in North American populations.
Male. Crown, nape, and back uniformly bright Prussian blue; upperwing, rump, uppertail coverts, and tail spectrum blue, the primaries and secondaries with inner webs and tips dusky blue to blackish; lores and eyeline anterior to eye dull mouse gray; sides of head below eyes and sides of throat chaetura drab washed blue; center of throat and upper breast muted russet or hazel; sides and flanks similar but slightly paler; lower belly, vent, and undertail coverts dull white; underwing coverts slate gray.
Female. Resembles Definitive Basic male but colors more muted; upperparts deep mouse gray washed blue; upperwing feathers and tail fuscous black tinged or washed blue; head and neck brown or chaetura drab washed blue or cinnamon rufous; chin and throat whitish to pale cinnamon, often with indistinct dusky malar streak; breast, sides, and flanks cinnamon or cinnamon rufous; underwing coverts spotted brownish.
Definitive Basic Plumage separated from Formative Plumage by having wing and tail feathers uniform in quality and freshness: greater coverts, tertials, and inner secondaries uniform in color and wear; primary coverts bright blue (male) or bluish (female), not contrasting in feather quality or color with greater coverts, the outermost primary covert relatively pointed and with thin or no whitish margins (Pyle 1997a: Fig. 231); basic outer primaries and rectrices broader, more truncate, bluer by sex, and relatively fresher (Pyle 1997a).
Plumage Structure And Coloration
Both female and male Eastern Bluebirds in Definitive Basic Plumage are more sexually dichromatic to each other than they appear to humans because they are able to detect color in the ultraviolet, making the UV-blue coloration potentially significant to between-male competition, perhaps female choice, and other aspects of bluebird social behavior including the behavior of adults to young (see Behavior)1.
Eastern Bluebirds are also sexually dimorphic in the feather structures that proximately control coloration as opposed to blue pigmentation (Shawkey et al. 2005). Feather barbs are made of layers of keratin, a spongy layer, and a layer of melanin granules that surround “large central vacuoles.” The nanostructure of the spongy layer predicts between-male variation in ultraviolet blue plumage color but it did not predict the principle color (hue) of the feathers (Shawkey al. 2003). Spectrometry (full spectrum) and transmission electron microscopy indicate that light scatters from the medulla of the barbs within the spongy layer, resulting in the ultraviolet blue feather coloration of bluebirds (Shawkey et al. 2005).
Variation in the ultraviolet blue coloration also occurs within the three species of Sialia, according to structure within the spongy layer of both upperpart and underpart feathers (Shawkey et al. 2003, 2005, 2006). These between-species shifts in pigments and microstructures are associated with evolutionarily significant morphological variation between closely related species; study is needed on the selective pressures that could have resulted in these species-specific changes. Experiments demonstrated that bacteria can degrade feather structures, but it appears from observational studies that bacterial load has little to no effect on feather hue among wild-living bluebirds (Shawkey et al. 2003, 2005, 2007).
Molt and plumage terminology follows Humphrey and Parkes (1959) as modified by Howell et al. (2003, 2004). Eastern Bluebird exhibits a Complex Basic Strategy (cf. Howell et al. 2003, Howell 2010), including complete prebasic molts and a partial-to-incomplete preformative molt but no prealternate molts (Stone 1896; Dwight 1900; Oberholser 1974; Pyle 1997a, 1997b; Figure 5).
Prejuvenile (First Prebasic) Molt
Complete, primarily May–Jul in North America, in the nest. Contour feathers develop by day 2; femoral-tract feathers begin emerging and feather shafts begin projecting from caudal and alar tracts by day 3; wings dark and rectrices just visible by day 4; feathers in crural region appear on day 5; upperwing secondary coverts break out of sheaths on day 8; capital feathers, secondaries, and rectrices break out of their sheaths and caudal feathers barely project beyond vent on day 9 (Donahue 1939, Pinkowski 1975a). Almost completely feathered by day 12; by day 14, unfeathered areas have disappeared, and rate of increase in wing length greatest; capable of weak, short-distance flight (PAG); on day 15, completely feathered. Achieves adult dimensions in wing chord and primary lengths around 35–40 d, about 2–3 wk after leaving nest. See Breeding: Young Birds for more information on timing of Prejuvenile Molt relative to physiological growth of chicks.
"First Prebasic" or "Prebasic I" Molt of Humphrey and Parkes (1959) and some later authors; see revision by Howell et al. (2003). Partial to incomplete, Jul–Oct in North America (Figure 5), primarily on or near breeding grounds but can complete molt after departing natal territory (Pinkowski 1976c). Includes most or all body feathers: 3–10 upperwing inner greater and carpal coverts (ca. 27% of individuals replace all 10), often 1–3 tertials (in ca. 50% of individuals), occasionally s6, rarely s5, and 0 (in ca. 50% of individuals) to all 12 (in ca. 31% of individuals) rectrices (Pyle 1997a, 1997b). Timing and extent of molt varies with hatching date: spring-hatched juveniles molt when 2–3 mo old, from the latter part of Jul to first half of Sep, whereas summer-hatched juveniles molt at less than 2 mo old, during very late Aug to first half of Oct, underwent more-rapid molt than spring-hatched juveniles, and often began migration before molt completed (Pinkowski 1976c). Among captive bluebirds, Preformative Molt began at age 35–84 d and showed positive correlation between start date and both day length and age at onset, consistent with photoperiodic influence on timing of this molt; required 35–96 d (mean 50.5 d ± 12.4 SD, n = 25) to complete, replacement of rectrices occurred among earlier molting individuals. No sex-specific differences in onset or length of molt noted (Pinkowski 1976c).
Definitive Prebasic Molt
Complete, Jul–Oct (Figure 5), on or near breeding grounds; study needed on the relationship between breeding territories and molting grounds. Primaries replaced distally (p1 to p10), secondaries replaced proximally from s1 and, likely, proximally and distally from the central or innermost tertial (s8 or s9) as typical of passerines, and rectrices probably replaced distally (r1 to r6) on each side of tail, with some variation in sequence possible.
Bill And Gape
Hatchlings have pink bill and ocher yellow gape. Older nestlings have black bill and ocher yellow gape and mouth. Adults have black bill and yellow gape.
Dark brown to black in nestlings; black in adults.
Legs And Feet
In hatchlings, flesh-colored; in older nestlings and adults, black or slate black.
1Definitions: “hue” indicates the principal color of an
object; technically it is the wavelength of peak light reflectance. The
total amount of light that an object reflects is called “brightness”,
which is technically the sum of reflectances across a sample (such as a
feather) of wavelengths of light. “Chroma” is about the spectral purity
of color, that is how undiluted it is with white. “Spectral saturation”
is a measurement that decouples the effects of chroma and hue, as in an
analysis of UV-violet chroma, which is highly correlated with hue. Used
in this way spectral saturation is the measurement within a range on
either side of a hue as the proportion of light reflected.
Possible differences in linear measurements between populations are not based on definitive study (
Howell, T. R. (1965). New subspecies of birds from the lowland pine savanna of northeastern Nicaragua. Auk 82:438-464.
Webster, J. D. (1973b). Middle American races of the eastern bluebird. Auk 90:579-590.
Phillips, A. R. (1991). The known birds of North and Middle America, Part II: Bombycillidae; Sylviidae to Sturnidae; Vireonidae. Denver: A. R. Phillips.
Phillips 1991), and could be systematically and easily accumulated. Age and sex classes (see Figure 7) may vary in wing length, with some males having longer wings than females. Although males and females appear to be the same size, mass measured for breeding females after completion of incubation is consistently higher than male mass in populations in S. Carolina, Georgia, and Arkansas (PAG, JHP, S. Isenberg pers. comm.).
Gowaty, P. A. and J. H. Plissner (2015). Eastern Bluebird (Sialia sialis), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.381