A typical paruline warbler in terms of size (about 11–13 cm long and 6–9 g in mass, depending on season; see Table 1
), bright coloration, and limb morphology (Keast et al. 1995).
Easily distinguished in all plumages from all other species. Older male (≥ 2 yr, Definitive plumage) is glossy black with contrastingly bright salmon orange patches on base of outer rectrices and base of remiges, as well as on sides of breast; lower underparts (belly, vent, and undertail coverts) white. In fresh Basic plumage (early fall) the black feathers may be buff-tipped. Female generally light gray on head, gray to olive green on back, and whitish below with pale yellow (not salmon orange) patches on tail, wings, and sides. Hatch-year and second-year male (through end of first breeding season) similar to female, but with yellow or sometimes orange yellow patches on tail, wings, and especially sides of breast. In addition, these yearling males often have small, irregular patches of black feathers on head, breast, or back, the extent of which increases gradually (in most individuals) as worn feathers are replaced facultatively from time of First Basic Plumage (= “formative plumage” [Wolfe et al. 2010], at a few weeks of age) until Definitive Alternate plumage (= “first molt cycle alternative plumage”, at about 1 yr [Rohwer et al. 1983]).
First-year male and older female usually (but not always) have large yellow patch on inner vane of third rectrix that extends to margin of vane, whereas yearling female tends to have smaller patch of yellow not reaching margin of vane or none at all (first-year female; intermediate condition not safely aged as yearling) on inner vane of third rectrix (Pyle 1997c, Curson et al. 1994, TWS). In females, yellow in tail feathers does not change color systematically with age (Spellman et al. 1987).
The American Redstart is also distinct for its acrobatic fly-catching behavior and associated wing and tail “flash patterns,” convergently similar to Myioborus warblers, among others (see Systematics: Related Species, and Diet and Foraging: Feeding). When foraging most actively, tends to droop wings slightly below fanned tail.
American Redstarts 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 molt and plumage descriptions pertain to all populations. Little or no geographic or sex-specific variation in molt strategies has been reported (see Appearance: Molts).
The following is based on published descriptions of plumage (Dwight 1900c, Ridgway 1902, Oberholser 1974c, Cramp and Perrins 1994b, Curson et al. 1994, Dunn and Garrett 1997), and criteria for determining age and sex (Robbins 1964, Gray III 1973, Foy 1974, Pyle 1997c). Appearances of sexes differ slightly in Juvenile and early Formative Plumages, moderately in late Formative or First Alternate plumages, and markedly in Definitive Basic and Definitive Alternate plumages; Definitive Plumage is assumed at Second Basic Plumage.
Present primarily June–July, in the nest. Hatchlings with small tufts of hair-brown natal down on crown and other parts of body (Dwight 1900c, Bent 1953b).
Juvenile (First Basic) Plumage
Present primarily June–July and only retained for a short period during and following fledging. Head, upperparts, and upperwing coverts deep sepia brown, the median and greater coverts with paler, wood-brown tips; chin and throat hair brown; sides of breast deep sepia brown; remainder of underparts white (see Plate 25 in Curson et al. 1994). Wings and tail as in Definitive Basic female except the yellow coloration to the tail (and to a lesser extent the wings) averages less extensive in females and can be slightly tinged salmon or orange in males (cf. Figure 275 in Pyle 1997c). Sexes similar except for average extent of yellow in the rectrices, in particular r3 (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 September–March or September–August, depending on existence or not of Prealternate Molts (see above), respectively.
Male. Plumage is similar to that of Definitive Basic female except patches on sides of breast (and sometimes patches in primaries and rectrices) often tinged orange, contrasting with lighter yellow underwing coverts; some later-replaced feathers, particularly among uppertail coverts occasionally washed orange to sepia. See Definitive Basic Plumage (below) for discussion of variation in coloration of Formative and Basic male remiges and rectrices plucked and regrown on the winter grounds.
Female. Similar to Definitive Basic female but rectrices average paler and with reduced amount of yellow in wings and tail, particularly on r3 (Figure 275 in Pyle 1997c).
In both sexes Formative Plumage is further distinguished from Definitive Basic Plumage by molt limits between replaced formative upperwing greater coverts, contrasting with older and browner retained juvenile primary coverts; retained juvenile outer primaries and rectrices thinner, more pointed, browner, and relatively more worn, although shape of rectrices does not differ as much as in most other wood-warblers (Pyle 1997c).
First Alternate Plumage
Alternate Plumages may not exist in American Redstart (see above) but, if so, they may be present primarily in March–August. First Alternate Plumage similar to Formative Plumage except most males acquire some black feathers on crown, face, back, throat, and/or breast. Acquisition of black feathers may begin as early as August as consequence of late Preformative Molt or adventitious feather loss and may continue throughout the non-breeding period (Rohwer et al. 1983). Thus, the male retains a largely female-like plumage during first reproductive season, until Second Prebasic Molt, at which time it acquires definitive plumage.
This phenomenon, often termed “delayed plumage maturation” (Procter-Gray and Holmes 1981, Rohwer et al. 1983), can be considered one form of heterochrony (different development rates), in which male sexual maturation precedes plumage maturation (Lawton and Lawton 1986). A variety of hypotheses have been proposed to explain delayed plumage maturation, and some have been tested in American Redstart (see Other: Life-history Traits: Delayed Plumage Maturation and Sexual Selection). Acquisition of black feathering in males as early as fall may suggest that it serves as an adaptation for social signaling on the overwintering grounds (Rohwer et al. 1983), or it could simply indicate that hormonal plumage-deposition mechanisms begin to operate in fall, as appears the case in other migrant passerines that exhibit delayed plumage maturation. The suggestion that first-spring males may acquire more black in western than in eastern populations of the American Redstart (Rohwer et al. 1983) could relate to differences in the timing of molt or hormonal, molt/color deposition cycles between populations (see Plumages: Definitive Basic Plumage).
Definitive Basic Plumage
Present primarily September–March or September–August, depending on existence or not of Prealternate Molts (see above), respectively.
Male. Head and upperparts uniform black, the feathers often with faint buffy margins when fresh (TWS); tail black, the basal half of outer rectrices (r2–r6 or r3–r6) orange, saturn red, or salmon pink. Upperwing coverts and remiges black, with orange or salmon basal portions of all remiges except innermost 2 tertials (s8–s9), forming large wing patches, the orange at the base of the primaries usually extending well beyond the tips of the primary coverts. Chin, throat, and center of breast black; sides of breast with large orange or salmon patches, edged ventrally and posteriorly by strip of black; belly and undertail coverts white; underwing coverts pale orange, the primary coverts mottled white, combining with bases of remiges to form a largely orange underwing. Amount and position of black feathers on breast ("bib") vary enough to allow recognition of individuals (Ficken 1962a, Lemon et al. 1992, Sherry and Holmes 1992b).
The degree of red-chroma saturation in rectrices of definitive male American Redstarts is variable, and was thought to relate to molt-migration in successful breeders of this species (Norris et al. 2004b), but is more likely associated with adventitious loss of feathers in overwintering areas, and regrowth of feathers with lower red-chroma (yellower) saturation (Reudink et al. 2008). Alternatively, it may relate to later molt timing in birds that successfully breed, a result of poorly known hormonal, molt/color deposition interactions, which may also be influenced by carotenoid availability in the diet at the time red-colored feathers are replaced (PP; see below).
To identify impacts of overwintering habitat and individual condition on feather color, Tonra et al. 2014 sampled feathers in January–March plus regrown feathers from the same individual birds at least a month later while still in overwintering areas. In general, neither habitat quality, circulating blood testosterone, nor energetic condition improved feather quality, and regrown feathers, particularly of Definitive Plumage males, tended to be less colorful in terms of color brightness (total reflected light), redness chroma (= purity of reflected color wavelength), and hue (dominance by one wavelength). Regrown remiges of Definitive Basic, but not Formative males, were less bright, for unknown reasons, but a possible consequence is reduced reproductive success (see Other: Life-history Traits: Delayed Plumage Maturation and Sexual Selection); and 10–22% of males in the same Jamaican population replaced remiges under natural conditions suggesting that this overwinter effect on plumage could be important ( Tonra et al. 2014 ). The reduced redness of replaced remiges occurred in both age classes of males, and its correlation with carotenoid pigment concentration suggested two possible explanations that remain to be tested: Either fewer high-carotenoid foods like lepidopteran caterpillars are consumed by redstarts in overwinter and/or more of the carotenoids are allocated to immune system function in winter. Formative, but not Definitive Basic males experienced increased hue of regrown remiges, representing a shift from the yellowish color typical of Juvenile remiges and rectrices in males to redder color typical of Definitive Basic Plumage; also, Formative males regrew body feathers black in parts of the plumage where the Definitive Basic Plumage is black. These color changes suggest a shift in Formative males sometime after fledging in the ability either to synthesize canthaxanthin pigment, or create it from beta-carotene; and in turn this suggests that selection for female-like plumage is stronger earlier than later in the non-breeding season of Formative males.
Female. Crown, nape, auriculars, and sides of neck medium gray, contrasting slightly to moderately with grayish-olive to olive back, rump, and uppertail coverts; tail pattern similar to that of male but rectrices brownish black with yellow to orangish-yellow bases to outer feathers, which are less extensive than the deep orange bases of Definitive Basic male but averaging more extensive than those of Juvenile female, especially on r3 (Figure 275 in Pyle 1997c). Dusky lores and very indistinct eye line present, usually bordered above by indistinct, thin white stripe that merges with supercilium and forms partial to near-complete white eye ring. Upperwing secondary coverts and remiges dusky margined olive, the tips to the median and greater coverts sometimes slightly paler, and the bases to the primaries and inner secondaries with yellow patch restricted to visible portions of inner secondaries (s1–s6 or s2–s6) and sometimes inner primaries (p1 or p1–p2); yellow of primaries usually does not extend beyond primary coverts. Underparts whitish with yellowish patch along side of upper breast; underwing coverts pale lemon yellow, mottled whitish toward primary coverts. Among females, Definitive Basic Plumage best separated from Formative Plumage by having wing and tail feathers uniform in quality and freshness, the primary coverts duskier, with fresher olive margins, and not contrasting in feather quality with greater coverts; basic outer primaries and rectrices broader, more truncate, duskier, relatively fresher, and averaging more extensive yellow, especially to r3 (Pyle 1997c).
As in males, the color of the bases of the rectrices can vary in females, between yellow and orangish yellow (Spellman et al. 1987). Differences appear not to be age specific but could relate to molt timing, molt/color deposition interactions, and/or carotenoid availability in the diet (see above). Females with more orange in the rectrices more often breed with older males, suggesting a mate-selection advantage to this trait (Spellman et al. 1987).
Definitive Alternate Plumage
Alternate plumages may not exist in the American Redstart (see above) but, if so, they may be present primarily in March–August. If present, Definitive Alternate Plumage essentially indistinguishable from Definitive Basic Plumage in each sex.
Molt and plumage terminology follows Humphrey and Parkes 1959, as modified by Howell et al. 2003 and Howell et al. 2004. American Redstart has been reported to exhibit 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, Dwight 1900c, Petrides 1943, Bent 1953b, Oberholser 1974c, Rohwer et al. 1983, Cramp and Perrins 1994b, Curson et al. 1994, Dunn and Garrett 1997, Pyle 1997c, Pyle 1997d; Figure 3). However, it is possible that feather replacement in October–May can be entirely attributed to adventitious feather loss and not to molt per se (see Molts: Prealternate Molts). If prealternate molts are absent, American Redstart would exhibit the Complex Basic Strategy (Howell et al. 2003).
Prejuvenile (First Prebasic) Molt
Complete, primarily June–July in North America, in the nest. Wing-feather papillae appear at Day 3; flight-feathers break through sheathes at Day 6; Juvenile feathering well developed by fledging at Days 8–10 (Bent 1953b). At this time, wing and tail about half grown (TWS). In one captive nestling the yellow portions of remiges became completely exposed at Day 26 and those of the tail at Day 20 (Petrides 1943), but growth may have been slowed by captive condition.
"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–September (Figure 3), commencing before or shortly after fledging, probably completing before southbound migration, but possibly continuing through early October in overwintering areas (see Molts: Prealternate Molts). One captive bird commenced this molt by Day 22, before completion of prejuvenile molt (Petrides 1943). Includes most or all body feathers and upperwing secondary coverts, and often the greater alula, but no primary coverts, primaries, secondaries, or rectrices (Pyle 1997c, Pyle 1997d).
First and Definitive Prealternate Molts
A limited prealternate molt has been reported for this species. In Venezuela, a high proportion of individuals of all age/sex groups reported to be molting a few body feathers in October–April (Lefebvre et al. 1992). This feather replacement, resulting in slow acquisition of black feathers in first-winter males (in same locations as black feathering in Definitive Basic males), may result from protracted Preformative Molt in October and/or has been attributed to adventitious feather loss (Rohwer et al. 1983). Apparently more-consistent molt of rictal bristles and feathers of the loral and orbital areas has been attributed to a prealternate molt (Rohwer et al. 1983); however, the timing of this feather replacement appears inconsistent between age/sex groups, which may indicate that this also represents adventitious loss. In an aerial forager like the American Redstart, it is possible that rictal bristles and feathers of the loral region are subject to frequent accidental loss. Constraints on plumage characteristics during the overwintering period (see Plumages: Definitive Basic Plumage) may explain why American Redstarts undergo a single, annual complete prebasic molt, in contrast to many other Setophaga warblers, which also undergo a partial prealternate molt during winter to attain colorful alternate plumage (Tonra et al. 2013). However, further study is needed on the winter-replacement of body feathers in many Neotropical migrant passerines.
Definitive Prebasic Molt
Complete, primarily July–September (Figure 3), on or near breeding grounds, although study needed on the relationship between breeding territories and molting grounds. Migration to molt rectrices up to 2,000 km south of breeding grounds (Norris et al. 2004b) has been questioned (Reudink et al. 2008; see below); however, a high incidence of body feather molt in October on the overwintering grounds (Lefebvre et al. 1992) may indicate protraction of body molt in some individuals (but see Molts: Prealternate Molts). On breeding grounds, adults can begin Definitive Prebasic Molt while still feeding dependent fledglings (Debruyne et al. 2006, TWS), as early as last few days of June and continuing into late August in Vermont (C. Rimmer, personal communication), and from early July to early September in Ontario, Canada (Debruyne et al. 2006). 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. Complete molting of remiges and rectrices requires about 6–7 wk; males generally begin a week or so earlier than females, and one-year-old males tend to begin earlier than older males (C. Rimmer, personal communication; see also Figure 2 in Debruyne et al. 2006). These age-related differences may relate to breeding success and failure, respectively. Older individuals show greater intensity of body molt than yearling birds later in the molt cycle, 25 August–10 September, perhaps because care of dependent fledglings is no longer a constraint (Debruyne et al. 2006).
Aspects of the Definitive Prebasic Molt are dependent on condition (Norris et al. 2005, Norris et al. 2007), but the timing and location of molt is controversial. Norris et al. 2007 argued that carotenoid-based remex coloration varies with molt location in North America, presumably at least partly due to diet: Midwestern and northeastern breeders tend to have redder chroma (hue) in these remiges. Further, male American Redstarts that molt south of the breeding grounds are more likely to do so in response to later, more demanding reproductive effort, and tend to grow remiges with reduced carotenoid content (i.e., yellower tail feather patches) during migration (Norris et al. 2004b). However, this was disputed by Reudink et al. 2008 (see also Debruyne et al. 2006), who showed that the results of Norris et al. 2004b could be explained by misinterpreting individual birds as molt-migrants within southern portions of the breeding range (high stable-hydrogen isotope ratio), while remex feather signals had instead resulted from facultative replacement of feathers on the overwintering grounds. Reudink et al. 2008 also suggested that the Norris et al. 2004b claim of a breeding effort-molt tradeoff may have resulted instead from an intervening (overwintering ground) effect, namely a tendency for males with high reproductive success to overwinter disproportionately in high quality territories, maintain body mass, and be more likely to replace inadvertently lost remiges with feathers containing a southern (Caribbean) stable-hydrogen isotope signal.