A typical Setophaga warbler, averaging about 13 cm in length and weighing 9–10 g (see Measurements). The male has a dark blue back, black face, throat, and flanks, whitish breast and belly, and a large white wing-patch. The female is greenish-gray in color, usually with a small, distinct white wing-patch, and a whitish or cream-colored supercilium and crescent-shaped mark below the eye. Males in their first year tend to have a greenish tinge to their dorsal tract feathers, and their alula covert is edged with green. With experience, observers can use these plumage characteristics in the field to distinguish first year from older males, with some certainty (Graves 1997a). In first-year females, the white wing-patch is often small in area, or in some cases lacking; the wing patch of older females is larger than younger females, though there is considerable overlap (Pyle 1997c, RTH).
Males in all plumages are unmistakeable, distinguished from all other species by their black, blue and white patterning and (usually) large white wing patches. Females can be separated from females of other warbler species by the presence of a white wing patch (sometimes small) in combination with unstreaked underparts, pale undertail coverts, yellowish/white eyebrow, dark cheek and a lack of wing bars. The female Black-throated Blue Warbler’s white wing patch easily distinguishes it from female Tennessee Warblers (Oreothlypis peregrina) and female Orange-crowned Warblers (O. celata), which are similar in coloration.
Black-throated Blue Warblers have 9 functional primaries (numbered distally, p1 to p10), 9 secondaries (numbered proximally, s1 to s9, including 3 tertials, s7 to s9), and 12 rectrices (numbered distally, r1 to r6, on each side of the tail). Geographic variation in appearance is slight, at best (see Systematics: Geographic Variation); the following appearance descriptions pertain to all populations. No geographic or sex-specific variation in molt strategies have been reported.
The following is based primarily on detailed descriptions of plumage (Dwight 1900c, Ridgway 1902, Forbush 1929, Harding 1931, Bent 1953b, Oberholser 1974c, Parkes 1979b, Curson et al. 1994, Dunn and Garrett 1997) and specific age- and sex-related criteria (Robbins 1964, Graves 1997a, Pyle 1997c). Unlike many other Setophaga warblers, appearances of sexes differs in Juvenile Plumage and are markedly different subsequent to this plumage. Definitive Plumage is assumed at Second Basic Plumage.
Present primarily June–July, in the nest. Hatchlings with gray tufts of down on capital, spinal, crural, caudal, and alar tracts (Harding 1931).
Juvenile (First Basic) Plumage
Present primarily July–August.
Male. Upperparts olive brown; remiges and rectrices blackish edged bluish gray, the bases of primaries white forming a white patch that typically extends 5–11 mm beyond the tip of the longest upperwing primary covert (Pyle 1997c), and the outer rectrices with subterminal white blotches edged with bluish plumbeous gray. Upperwing coverts, secondaries, and tertials edged olive green tinged bluish. Lores and submalar stripes dusky; short whitish to straw-colored superciliary stripe; throat and upper breast buffy olive; chin and lower breast, abdomen and undertail coverts chamois (color 123D of Smithe 1975).
Female. Similar to Juvenile male, but remiges and rectrices browner and edged duller greenish, without bluish; basal white patch on primaries reduced distally, extending 0–5 mm beyond tips of greater primary coverts, sometimes being concealed by greater primary coverts, and often duller white (Forbush 1929, Pyle 1997c); outermost rectrix (rarely r5 also) with pale, rarely whitish, indistinct terminal spot on inner web; superciliary stripe may be nearly absent (Parkes 1979b). Sex-specific formative body feathering emerges quickly after fledging (Pyle 1997c).
"First Basic" or "Basic I" plumage of Humphrey and Parkes 1959 and later authors; see revision by Howell et al. 2003. Present primarily July–March (or through second July if First Prealternate Molt is absent).
Male. Similar to Definitive Basic male but averages duller; bluish upperpart feathers edged olive-green when fresh; black of head and throat averages slightly duller, less contrasting, and often mottled ashy gray or whitish and with a few whitish feathers on lower part of eyering; most to all upperwing secondary coverts replaced, black with distinct plumbeous gray edging, contrasting with duller brownish-black primary coverts with dull olive edging; primaries and rectrices duller brownish black, thinner, more pointed, and relatively more worn, the basal patch to the primaries less distinct and reduced (extending 5–11 mm beyond the tip of the longest upperwing primary covert), and the outer rectrices (r5–r6) averaging smaller and less-distinct white patches (Graves 1997a, Pyle 1997c). See Graves 1997a for description of colorimetric differences of wing between Formative and Definitive Basic males based on spectrophotometric measurements.
Female. Similar to Definitive Basic female but duller; upperparts with little or no bluish tinge; superciliary stripe yellowish or may be nearly absent (and less conspicuous in the field; Parkes 1979b); lower eyelid with less white; primaries and rectrices duller brownish, thinner, more pointed, and relatively more worn, the bases to the primaries with less white (concealed by primary coverts or extending 1–6 mm distal to longest primary covert), the bases of the rectrices without bluish-green edging and the outer rectrices with little or no whitish (sometimes, inner web of r6 and rarely r5 with pale indistinct terminal spot).
First and Definitive Alternate Plumages
If these plumages occur (see First and Definitive Prealternate molts), present primarily March–August and very similar if not indistinguishable from Formative and Definitive Basic plumages, respectively, within each sex. Some males may acquire some blue-gray feathers to crown and chin. Wear of feather fringing results in upperparts of males becoming bluer and in females becoming more olive (less green) on upperparts and whiter on underparts. Criteria to separate First Alternate from Definitive Alternate plumages similar to that described under Formative and Definitive Basic plumages, the molt contrast in the upperwing coverts often becoming more pronounced due to increased degradation rate of retained juvenile feathers compared with replaced formative feathers.
Definitive Basic Plumage
Present primarily August–March (or through July if Definitive Prealternate Molt is absent).
Male. Upperparts bright bluish-plumbeous gray, the crown feathers with dusky shaft streaks and some back feathers with black central spots; overall the feathers fringed with duller greenish blue when fresh; tail and upperwing feathers uniformly black edged with blue-gray, the primaries with extensive and distinct white bases forming patch that extends 7–14 mm distal to tip of longest primary covert and the outer three rectrices (r4–r6) with distinct subterminal white patches. Lores, supraloral area, auriculars, chin, throat, and sides of breast and flanks uniformly deep black, somewhat broken along flanks by white streaking; remainder of underparts including axillars and underwing coverts white.
Female. Upperparts olive to olive-green, the crown, rump, and uppertail coverts often tinged bluish; tail and upperwing feathers uniformly dusky edged olive, some marginal lesser wing coverts tinged bluish, the primaries with white bases forming patch that extends 4–8 mm distal to tip of longest primary covert, the bases to the rectrices often tinged bluish green, and and the outer two rectrices (r5–r6) usually with indistinct subterminal pale to whitish patches. Lores and auriculars olive, sometimes tinged bluish gray; supercilium whitish, widening over auriculars; feathers below eye whitish forming crescent-shaped mark; underparts buffy yellowish to olive, paler whitish on throat and variably darker olive on sides and flanks; wing linings whitish.
In both sexes, Definitive Basic Plumage separated from Formative Plumage by having wing and tail feathers uniform in quality and freshness, the primary coverts blacker (male) or duskier (female), edged blue (male) or olive (female), and not contrasting in feather quality with greater coverts; basic outer primaries and rectrices broader, more truncate, blacker, relatively fresher, and averaging larger and more-distinct white patches to r5–r6, sex for sex (Pyle 1997c).
One report of a probable bilateral gynandromorph in which left side was male-like and right side was female-like in plumage characteristics (Patten 1993a). Analyses of photos of this individual indicated the left side was similar to first basic plumage of a male, while right side was more of a mosaic of male and female plumages, an unusual pattern for gynandromorphic passerines (Graves et al. 1996). A partial albino female bred and was photographed at Hubbard Brook Experimental Forest in 1996 (RTH, NLR, TSS).
Molt and plumage terminology follows Humphrey and Parkes 1959, as modified by Howell et al. 2003 and Howell et al. 2004. The Black-throated Blue Warbler apparently exhibits a Complex Alternate Strategy (cf. Howell et al. 2003, Howell 2010b), including complete prebasic molts, a partial preformative molt, and limited prealternate molts in both first and definitive cycles (Stone 1896, Stone 1901, Dwight 1900c, Oberholser 1974c, Curson et al. 1994, Dunn and Garrett 1997, Pyle 1997d, Pyle 1997c; Figure 2). However, the presence of a Prealternate Molt in this species may require verification (see below); if absent, this species would exhibit the Complex Basic Strategy (Howell et al. 2003).
Prejuvenile (First Prebasic) Molt
Complete, primarily June–July in North America, in the nest. According to Harding 1931, primaries break sheaths by day 4, and are 10 mm long by day 5. At Hubbard Brook in New Hampshire, data indicate primaries usually break sheaths by day 5 or 6 and most to all down lost by day 7 (NLR, TSS). Young can fly weakly when they leave the nest on day 8 or 9 (RTH); growth of primaries and rectrices completed after fledging, probably by day 10–12, at which time Preformative Molt of body feathers has often begun.
"First Prebasic" or "Prebasic I" Molt of Humphrey and Parkes 1959 and some later authors; see revision by Howell et al. 2003. Partial, primarily June–August (Figure 2), on breeding grounds. Commences during or shortly after period of fledging. Includes most or all body feathers and upperwing secondary coverts (outer two greater coverts can be retained in about 10% of individuals), sometimes the greater alula and rarely (in about 3% of individuals) 1–2 tertials, but no primary coverts, primaries, outer secondaries, or rectrices (Pyle 1997d, Pyle 1997c).
First and Definitive Prealternate Molts
Prealternate molts reported to be absent or limited to some body feathers (restricted to head and chin), primarily in March–May (Figure 2), on non-breeding grounds (Pyle 1997c). However, confirmation that spring feather replacement is based on separate inserted molts may need to be verified; it is possible that protracted and/or suspended Preformative and/or Definitive Prebasic molts, or adventitious feather replacement, may occur on non-breeding grounds and have been mistaken for separate Prealternate Molts, but if no follicles are activated twice during molt, only one a single (Preformative or Definitive Prebasic) molt exists. Reports that Prealternate Molts can begin as early as October (Pyle 1997c) may also involve protracted molts or adventitious feather replacement. More study of Prealternate molt is needed.
Definitive Prebasic Molt
Complete, primarily July–September (Figure 2), on or near breeding grounds, although study needed on the relationship between breeding territories and molting grounds. Primaries replaced distally (p1 to p9), secondaries likely replaced proximally from s1 and 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
Mandibles straw-colored at 7 d becoming black in adults. Lining of mouth “ochre” at hatching becoming pink (color 7; see Smithe 1975) by day 7.
Blackish brown at day 7 and in adult.
Legs and Feet
Flesh-colored at day 7, becoming sepia (Forbush 1929) to black in adults.