Swainson's Warbler

Limnothlypis swainsonii

Order:
Passeriformes
Family:
Parulidae
Sections

Appearance

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Swainson's Warbler (10 April).

A medium-sized, plain wood-warbler with a long, heavy, sharply pointed bill. Upperparts, including wings, are brown or olive-brown, with the crown reddish brown. A prominent white or pale yellow superciliary is present, and lores and postocular streak are brownish. The underparts are yellowish white to white with no streaking. Sexes are similar in appearance. There are no prealternate molts and only slight seasonal variation in appearance due to feather wear.

© Irvin Pitts , South Carolina , United States , 10 April 2018
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Figure 2. Annual cycle of molt, breeding, and migration of Swainson's Warbler.

Data represent molt, breeding, and migration of Swainson’s Warbler in the southeastern United States; birds from higher latitudes and higher elevation begin breeding later in spring than southern birds do. Thick lines show peak activity, thin lines off-peak.

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Juvenile Swainson's Warbler undergoing Preformative Molt (7 July).

Juvenile Plumage is similar to Definitive Basic Plumage, but upperparts and flanks are plainer brown. Supercilium and postocular stripe are indistinct or lacking; upperwing median and greater coverts tipped buff to cinnamon-brown, forming indistinct wing bars. Central underparts more or less clouded with brown. Body feathers (especially undertail coverts) more filamentous than in later plumages due to lower barb density.

© Robert Bochenek , North Carolina , United States , 7 July 2018
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Swainson's Warbler undergoing Preformative Molt (26 June).

Bill is pale pinkish buff in hatchlings, becoming darker with age. The Preformative Molt occurs rapidly, completing during and shortly after fledging, and usually includes all body feathers but no remiges or rectrices.

© Nick Anich , Arkansas , United States , 26 June 2006
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Swainson's Warbler undergoing Preformative Molt (26 June).

Partial molt; usually includes all body feathers and secondary coverts but no alula feathers, remiges, or rectrices. Note difference between rust-toned juvenile feathers and new olive-toned formative feathers.

© Nick Anich , Arkansas , United States , 26 June 2006
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Formative Swainson's Warbler (11 November).

Very similar to Definitive Basic Plumage and often difficult to distinguish. Best identified 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. Here, the worn and dull juvenile alula is retained and the overall worn look to the primaries and rectrices in November indicates Formative Plumage.

© Denny Swaby , North Side , Cayman Islands , 11 November 2017
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Formative Swainson's Warbler (6 May).

The retained brown primary coverts contrast with the replaced olive-brown secondary coverts, and the rather worn primary and rectrix tips indicate Formative Plumage.

© Neil Hayward , Massachusetts , United States , 6 May 2018
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Definitive Basic Swainson's Warbler (14 April).

Crown plain brown, often tinged reddish. Sides of head pale buffy brownish with pale supercilium, dusky triangular spot of in front of eye, and brownish postocular streak. Remaining upperparts and upperwing coverts plain olive; tail plain olive-brown; tertials warmer brown; secondaries and primaries dusky, edged with light brown or olive. Underparts pale dull yellowish or yellowish white, shaded with olive or olive-grayish laterally. Note the lack of contrast between the primary and greater coverts and the very broad primary tips, indicating Definitive Basic Plumage.

© Mark R Johnson , Virginia , United States , 14 April 2012
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Definitive Basic Swainson's Warbler (21 October).

Appearance in fall tends to be duller overall due to duller brown feather veiling; the crown is browner, less reddish, and the breast and flanks more or less clouded with grayish. As the veiling wears off the plumage becomes brighter by spring.

© Daniel Aldana , Escuintla , Guatemala , 21 October 2018
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The head and bill of a male Swainson's Warbler (14 May).

The bill is grayish pink with undersurface of the mandible flesh-colored; the iris is brown. Sexes are alike in plumage and similar in measurements but birds in the hand during the breeding season can be sexed by the presence of a cloacal protuberance (in males) or a brood patch (in females).

© Nick Anich , Arkansas , United States , 14 May 2006

A plain, medium-sized wood-warbler with a long, heavy, sharply pointed bill. Total body length 130–140 mm; mass approximately 15 g. Sexes are similar in appearance and there are no alternate plumages and only slight seasonal variation in appearance due to feather wear. Upperparts, including wings, are brown or olive-brown, with the crown reddish brown. A prominent white or pale yellow superciliary is present, and lores and postocular streak are brownish. The underparts are yellowish white to white with no streaking (25, 1). The bill is grayish pink with undersurface of the mandible pinkish-brown; the iris is dark brown; and the legs and feet are pinkish. This species' foraging behavior—lifting and flipping fallen leaves (7)—is distinctive.

Recently fledged young are brown above, dirty white on underparts, lack a supercilium, and have pink legs and cream-colored gape flanges (NMA, TJB). Juvenile feathers may appear more cinnamon-brown than olive-brown in upperparts and wings, but this is not apparent in the field. Juvenile plumage is only present for a short time in summer, after which young (formative-plumaged) birds resemble adults.

Similar Species

Although relatively plain, Swainson's Warbler is generally readily identifiable when seen due to its unique plumage within its range and large bill. Worm-eating Warbler (Helmitheros vermivorum) is more olive above, buffier underneath, and shows prominent black stripes on the crown; Ovenbird (Seiurus aurocapilla), Northern Waterthrush (Parkesia noveboracensis), and Louisiana Waterthrush (P. motacilla) are streaked below; Bewick's Wren (Thryomanes bewickii) has a thinner bill, dark legs, and barring on a longer, narrower tail.

Because Swainson's Warbler is more often heard than seen, song is an important distinguishing characteristic. The first half of the primary song (loud, clear, downslurred notes) resembles the beginning notes of the Louisiana Waterthrush song, and the second half resembles the ending of the Hooded Warbler (Setophaga citrina) song. The chip note, a sharp tchick, is extremely similar to the first or last note of the White-eyed Vireo (Vireo griseus) song (1).

Detailed Description

Swainson's Warbler has 9 functional primaries (numbered distally, from innermost p1 to outermost p9), 9 secondaries (numbered proximally, s1–s9, and including 3 tertials, s7–s9 in passerines), and 12 rectrices (numbered distally, r1–r6, on each side of the tail). Geographic variation in appearance is slight at best (see Systematics: Geographic Variation). No geographic variation in molt strategies has been reported.

Plumages

The following is based primarily on detailed plumage descriptions of Ridgway (26), Bent (27), Oberholser (28), Curson et al. (29), and Dunn and Garrett (30); see Pyle (31) for age-related criteria. Sexes show similar aspects in all plumages, however, chromatic differences may be present from an avian perspective (32). No fluorescence was observed when 2 specimens of adults were subjected to a 395 nm ultraviolet light (P. Anich, personal communication). Definitive appearance is assumed following the Second Prebasic Molt. Colors in following are from Smithe (33); original color names from Ridgway (34) are in parentheses.

Natal Down

Present April–June. Young are naked and altricial when hatched. Oberholser (28) described the natal plumage as being “dark, rather brownish-drab.”

Juvenile (First Basic) Plumage

Present primarily May–July. Similar to Definitive Basic Plumage but upperparts and flanks plainer brown, varying from drab (broccoli) to olive-brown (bister); supercilium and postocular stripe indistinct or lacking; upperwing median and greater coverts tipped cinnamon-brown, forming indistinct wing bars; central underparts more or less clouded with brown. Body feathers (especially undertail coverts) more filamentous than in later plumages due to lower barb density. Juvenile Plumage in Swainson's Warbler and other wood-warblers is ephemeral and variation not well described (see 35).

Formative Plumage

"First Basic" or "Basic I" plumage of Humphrey and Parkes (36) and later authors; see revision by Howell et al. (37). Present primarily September–August. Very similar to Definitive Basic Plumage and often difficult to distinguish. Best identified 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, especially by spring (31). Occasionally up to 3 outer greater coverts can be retained, relatively worn and tipped pale.

Definitive Basic Plumage

Present primarily September–August. Crown plain brown, often tinged reddish, varying from deep hair brown or olive-brown to raw umber (mummy brown), sometimes with very indistinct paler median streak; sides of head pale buffy brownish with narrow light yellowish-buff to whitish supercilium, dusky triangular spot of in front of eye, and brownish postocular streak. Back, scapulars, rump, uppertail coverts, and upperwing coverts plain olive; tail plain olive-brown; tertials warmer brown, inclining to raw umber (mummy brown) or Prout's brown; secondaries and primaries dusky, edged with light brown or olive. Underparts pale dull yellowish or yellowish white, shaded with olive or olive-grayish laterally; underwing coverts white, some feathers tinged yellow. Plumages appear slightly different in spring and fall due to affects of feather wear: fall plumage tends to be duller overall (feathers veiled with duller brown), the crown browner, less reddish, and the breast and flanks more or less clouded with grayish.

Definitive Basic Plumage separated from Formative Plumage by having upperwing and tail feathers uniform in quality and freshness, the primary coverts darker brown and not contrasting in feather quality with greater coverts; basic outer primaries and rectrices broader, more truncate, blacker, and relatively fresher (31).

Aberrant Plumages

Miller (38) found partial albinism in 2 of 4 nestlings banded from a nest in South Carolina.

Molts

Molt and plumage terminology follows Humphrey and Parkes (36), as modified by Howell et al. (37, 39). Swainson's Warbler exhibits a Complex Basic Strategy (cf. 37, 40), including complete prebasic molts and a partial preformative molt but no prealternate molts (41, 28, 29, 30, 31, 42; Figure 2).

Prejuvenile (First Prebasic) Molt

Complete, primarily May–July, in the nest. Pennaceous feathers emerge on wings on day 4; lesser and median coverts are out of feather sheaths at day 8; and most wing feathers are out of sheaths with head fully feathered and having 2 pronounced tufts of down at day 9 when the nestling is ready to fledge (43). Duration of Prejuvenile Molt among individuals likely resembles that of other warblers in being ca. 5–6 d in duration; presumably completed or near-completed by fledging at days 9–11, with final growth of rectrices occurring 3–4 or more days post fledging.

Preformative Molt

"First Prebasic" or "Prebasic I" Molt of Humphrey and Parkes (36) and some later authors; see revision by Howell et al. (37). Partial, primarily May–July; can begin in the nest and completes on breeding grounds. Usually includes all body feathers and secondary coverts (up to 3 outer greater coverts occasionally retained) but no alula feathers, remiges, or rectrices (31, 42).

Definitive Prebasic Molt

Complete, primarily June–August, on or near breeding grounds although study needed on relationship between breeding territories and molting grounds (cf. Pyle [44]). Primaries replaced distally (p1 to p9), secondaries replaced proximally from s1 and proximally and distally from the central tertial (s8), and rectrices probably replaced distally (r1 to r6) on each side of tail, with some variation in sequence possible.

Bare Parts

Bill

Pale pinkish buff in hatchlings; grayish pink with undersurface of the mandible pinkish-brown in adults.

Iris

Dark brown.

Legs and Feet

Pale pinkish buff in hatchlings, and pinkish in adults.

Measurements

Linear Measurements

See Table 1. There are no significant differences in linear measurements of males and females. Bill and tarsus length are very similar between males and females; tail and wing chord length are generally greater in males than females, but there is considerable overlap in measurements.

Mass

See Table 2. Overall, there is no significant difference in body mass of males and females. During the breeding season, females are often heavier than males, likely due to the presence of partially developed eggs; in addition, female mass is more variable than that of males. Mass during the overwintering period appears comparable to values during the breeding period, but birds appear to be heavier during migration.

Recommended Citation

Anich, N. M., T. J. Benson, J. D. Brown, C. Roa, J. C. Bednarz, R. E. Brown, and J. G. Dickson (2019). Swainson's Warbler (Limnothlypis swainsonii), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.swawar.03