Connecticut Warbler Oporornis agilis Scientific name definitions

Jay Pitocchelli, Julie L. Jones, and David C. Jones
Version: 2.0 — Published June 2, 2023

Plumages, Molts, and Structure


The Connecticut Warbler has 9 functional primaries (numbered distally, from innermost p1 to outermost p9), 9 secondaries (numbered proximally, from outermost s1 to innermost s9, and including including 3 tertials, s7–s9 in passerines), and 12 rectrices (numbered distally on each side of the tail, from innermost r1 to outermost r6). No geographic variation in appearance (see Geographic Variation) or molt strategies reported.

The following is based primarily on detailed plumage descriptions of Dwight (5), Ridgway (6), Bent (7), Roberts (8), Oberholser (9), Curson et al. (10), and Dunn and Garrett (11), along with examination of Macaulay Library images; see Robbins (12), Pyle and Henderson (2), and Pyle (3) for specific age- and sex-related criteria. Appearances of the sexes similar in Juvenile Plumage, show only slight average differences in Formative Plumage, and show moderate differences in subsequent plumages. Definitive appearance is assumed at the Second Basic Plumage.

Natal Down

Mostly naked (13); down, if present, in the nest (June–July), is undescribed.

Juvenile (First Basic) Plumage

Present primarily June–July, in the nest or near the natal territory. Juvenile Plumage ephemeral and not well described; may resemble that of the Mourning Warbler (Geothlypis philadelphia) (see 14). Upperparts and head olive brown; eye ring buff (not white); underparts with olive-brown breast and flanks, merging with buff to yellowish belly. Sexes alike.

Formative Plumage

"First Basic" or "Basic I" plumage of Humphrey and Parkes (15) and later authors; see revision by Howell et al. (16). Present primarily August–March. Similar to Definitive Basic female (see below), except eye ring more creamy yellow than white. Hood of Female averages more olive-brown and less gray than that of Male, but overlap between sexes occurs. Some (primarily females) may have dull yellowish or mustard wash to face and throat. Formative Plumage further distinguished from Definitive Basic Plumage by duller and browner retained juvenile upperwing primary coverts, with reduced olive edging, contrasting with newer formative greater coverts, and retained juvenile outer primaries and rectrices thinner, more pointed, browner, and relatively more worn (3).

Rohwer and Butcher (17) believed the duller Formative Plumage of Oporornis to be an adaptation to predation avoidance during their first overwintering period because First Alternate Plumage in spring becomes brighter.

First Alternate Plumage

Present primarily March–August.

Female. Similar to Definitive Alternate female (see below) but duller, head with more brown and olive, less gray. Underparts duller yellow.

Male. Similar to Definitive Alternate male (see below) but head, throat, and breast duller gray, lower breast with little or no dark gray mottling.

In both sexes, molt limits between upperwing greater and primary coverts, as described under Formative Plumage, and retained juvenile rectrices and remiges become noticeably more worn than comparable feathers in Definitive Alternate Plumage, and remain useful for distinguishing age.

Definitive Basic Plumage

Present primarily September–March.

Female. Crown, head, throat, and breast olive to grayish olive, the feathers fringed dull olive when fresh, the throat and lores tinged buff to yellowish. Remainder of upperparts, upperwing coverts, and rectrices olive. Full cream to whitish eye ring present. Primary coverts, primaries, and secondaries dusky gray with olive outer webs. Remainder of underparts medium-dull greenish yellow to yellow, the flanks often washed olive. Underwing coverts mixed yellowish, brownish olive, and gray.

Male. Similar to Definitive Basic female except brighter overall, and head, throat, and breast dark gray, the feathers fringed brownish when fresh. Lower breast sometimes with darker gray mottling. Eye ring brighter white and more conspicuous. Lower underparts brighter yellow.

In both sexes, Definitive Basic Plumage separated from Formative Plumage by having duskier (less brownish) basic primary coverts with broader olive edging, not contrasting in feather quality with greater coverts, and basic outer primaries and rectrices broader, more truncate, duskier, and relatively fresher (18).

Definitive Alternate Plumage

Present primarily March–August.

Female. Similar to Definitive Basic Plumage but averages grayer head, throat, and breast, and brighter yellow underparts resulting from molt and the wearing off of olive feather fringing to the crown, nape, and breast feathers. Breast sometimes with diffuse darker streaks or mottling.

Male. Similar to Definitive Basic Plumage but brighter due to combination of molt and wearing of brownish olive feather fringing. Head becomes brighter gray to bluish gray. Darker gray to slate mottling to breast present on many birds, averages more extensive and conspicuous. Lower underparts and flanks become slightly brighter yellowish.

In both sexes, separated from First Alternate Plumage by the primary coverts, primaries, and rectrices, as described for Definitive Basic Plumage. Differences become more distinct due to greater contrasts in wear of First Alternate Plumage.


Molt and plumage terminology follows Humphrey and Parkes (15), as modified by Howell et al. (16, 19). Connecticut Warbler exhibits a Complex Alternate Strategy (cf. 16, 20), including complete prebasic molts, a partial preformative molt, and limited prealternate molts in both first and definitive cycles (21, 5, 7, 9, 2, 10, 11, 22, 3; Figure 1).

Prejuvenile (First Prebasic) Molt

Complete, June–July, in the nest. No information on timing or sequence of pennaceous feather irruption and development.

Preformative Molt

"First Prebasic" or "Prebasic I" Molt according to Humphrey and Parkes (15) and some later authors; see revision by Howell et al. (16). Partial, June–August (Figure 1), on breeding grounds, commencing and sometimes completing before fledging. Includes most or all body plumage and secondary coverts, but no alula, primary coverts, primaries, secondaries, or rectrices.

First Prealternate and Definitive Prealternate Molts

Partial, March–May (Figure 1), on or near nonbreeding grounds. A male in Definitive Alternate Plumage captured in southern Cuba in May showed no sign of molt, indicating that prealternate molt had completed before the beginning of spring migration (23). Restricted primarily to feathers of head, throat, and breast. The First and Definitive Prealternate molts may be similar in timing and extent although the First Prealternate Molt in males may be more extensive than Prealternate Molt of older males and females (10, 3) to accommodate more significant plumage change between Formative and First Alternate plumages.

Definitive Prebasic Molt

Complete, July–August (Figure 1), on or near breeding grounds, although study needed on the relationship between breeding territories and molting grounds (cf. 24). Primaries replaced distally (p1 to p9), secondaries replaced proximally from s1 and proximally and distally from the central tertial (s8), and rectrices generally replaced distally (r1 to r6) on each side of tail, with some variation in sequence possible and molt being rapid at times.

Bare Parts

Bill and Gape

In juveniles 9–10 days old, tomia yellow, inside of mouth pinkish orange (25). In adults, bill blackish with pinkish at base, which can be brighter during spring.


Very dark, perhaps blackish (JP).

Tarsi and Toes

Legs dusky pink (10); paler, light pinkish in juveniles (25).


Bird banders and museum curators have had problems distinguishing Mourning Warbler, MacGillivray's Warbler (Geothlypis tolmiei), and Connecticut Warbler because of their similar plumages. Lanyon and Bull (4) analyzed morphometrics of Mourning Warbler and MacGillivray's Warbler to determine usefulness of measurements for identifying specimens; Connecticut Warbler was included as an additional comparison of a closely related species with similar plumage. Their study contains the most extensive measurement data for this species. Intraspecific variation in mensural characters including geographic variation and age variation deserve further study.

Collins et al. (26) tested the hypothesis that body size of birds would decrease in size with increasing global temperatures and climate change, in accordance with Bergmann’s rule. They found positive changes in wing length and fat-free mass of fall migrants captured at Patuxent Wildlife Research Center, Laurel, Maryland, 1980–2012, but these increases were not significant (26).

Linear Measurements

Relative to Mourning Warbler and MacGillivray's Warbler, the Connecticut Warbler and Kentucky Warbler (Geothlypis formosa) spend more time on the ground, have longer legs, and are larger and heavier (see Table 1 and Table 2). Saulnier (27) found little sexual dimorphism in external measurements of breeding males and females from Quebec. There were no significant differences between the sexes for wing length, culmen, and tail length, but females had longer undertail coverts.

Bill Length

From Ridgway (6): male 11.9 mm (range 11.4–12.4, n = 7); female 11.8 mm (range 11.7–11.9, n = 5). From Curson et al. (10): male 11.4–12.5 mm (n = 10); female 11.5–12.5 mm (n = 10). From Saulnier (27): male 11.27 mm (range 9.57–12.60, n = 31); female 11.25 (9.13–12.62, n = 9).

Wing and Tail Length

Table 1. Data reported here for Connecticut Warbler are flattened wing length, tail length, and flattened wing minus tail (W–T) and come from Lanyon and Bull (4). Wing of male Connecticut Warbler is longer than that of male Mourning Warbler or MacGillivray's Warbler. Tail is shorter than that of MacGillivray's Warbler but similar to that of Mourning Warbler (see Table 1).

Wing length–Tail length (W–T) is the most useful metric for distinguishing specimens of Mourning Warbler, MacGillivray's Warbler, and Connecticut Warbler (4, 1). This difference is greatest in Connecticut Warbler, smallest in MacGillivray's Warbler, and intermediate for Mourning Warbler (see Table 1); there is some overlap between Mourning Warbler and MacGillivray's Warbler, but neither of these species overlap with Connecticut Warbler.

Tarsus Length

From Ridgway (6): male 21.3 mm (range 20.6–23.1, n = 7); female 20.8 mm (range 19.0–21.8, n = 5). From Curson et al. (10): male 20.0–23.1 mm (n = 10); female 19–22 mm (n = 10).

Skeletal measurements also show that Connecticut Warbler is larger than Mourning Warbler and MacGillivray's Warbler, but about same size as Kentucky Warbler (Geothlypis formosa) (1; JP, unpublished data; Table 2). Connecticut Warbler has larger skull, larger girdle, and longer leg and wing bones than Mourning Warbler and MacGillivray's Warbler. The longer leg bones of Connecticut Warbler may be an adaptation for walking on the ground, whereas Mourning Warbler and MacGillivray's Warbler have shorter legs and hop through dense vegetation.


Saulnier (27) found significant differences in mass of males and females measured during the breeding period: males 13.6 g (range 12.3–15.7, n = 31); females 14.6 g (10.3–17.3, n = 9); paired males were 0.9 g heavier than unpaired males. Mean mass of fall migrants in western Pennsylvania was 13.3 g (range 11.2–15.5, n = 55) (28). A migrant banded in Peru weighed 12.5 g (29).

Recommended Citation

Pitocchelli, J., J. L. Jones, and D. Jones (2023). Connecticut Warbler (Oporornis agilis), version 2.0. In Birds of the World (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.conwar.02