Connecticut Warbler Oporornis agilis Scientific name definitions

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



The Connecticut Warbler is a socially monogamous breeder with one clutch per breeding season. Only the females incubates the eggs, but both male and female participate in brooding, feeding young birds and nest sanitation. Little is known about the extent of extra-pair copulations.


Winger and Pegan (52) calculated an average inter-migratory period of 108.5 d that birds spend on the breeding grounds. They based this calculation on eBird data and the median peak fall and spring migrations through Chicago, Illinois from 2000–2017. Incubation by females began from 20–24 June in Quebec (27).

Pair Formation

Courtship and mating begin soon after arrival on breeding grounds in late May, continue into late June throughout breeding range (Figure 1), and, in Michigan, coincide with peak singing and territoriality (25).

Nest Building

Most nesting accounts reported nesting in June; there are occasional records from July (Figure 1).

First Brood

Nesting dates are highly variable throughout the breeding range. Most egg dates (range-wide) have been recorded from mid to late June (7), mid to late June in British Columbia (91), 15–30 June in Alberta, some later records in mid-July (85), early June–early July in Wisconsin (102), and first week of June through early July in Michigan (25, 129). Some specific dates for nests with eggs were summarized by Kilgore (221): 27 May in Michigan, 7 June, in Ontario, 12 June and 21 June in Manitoba, 13 June in Minnesota, 15 and 19 June in Alberta. Nests with eggs have also been reported on 8 June in Wisconsin (102) and 18 June in Minnesota (13). Nests with young have occurred on 30 June in Minnesota (221), 1 July and 27 July in Michigan (109, 222), 7 July in Wisconsin (102). Fledglings observed in July to as late as 10 August in Michigan (109), 24 July and 15 August in Ontario (223), 6 July–27 July in Wisconsin (102). These dates suggest that nesting extends well into July, perhaps re-nesting. There is no information on the extent of parental feeding and protection after young depart the nest; deserves study.

Second/Later Broods

In a thorough study of several breeding pairs by Saulnier (27) in Quebec, there was no mention of birds attempting a second brood; “probably” only a single brood in British Columbia (89). A single brood per year is most likely across the breeding range, but deserves further attention.

Nest Site

Selection Process

Information needed.

Site Characteristics

Nest site characteristics differ between the eastern and extreme western parts of the breeding range. In the eastern portion of the breeding range, the nest site is similar to that of other bog-nesting warblers, such as Palm Warbler (Setophaga palmarum) and Nashville Warbler (Leiothlypis ruficapilla) (25). The nest is hidden on or near the ground, in thick undergrowth of saplings, among thickets, or at the base of a shrub, in a sunken clump or mound of moss, or in dry grasses sometimes covered by overhanging vegetation (83, 85, 224). Western birds tend to breed in well-drained upland sites in or near aspen forests (7, 205). Nests in open forests with widely spaced trees, such as aspen (Populus) and balsam (Abies) (199) or willows (Salix) (7). In Ontario, nests have been found well concealed in raspberry (Rubus) thickets in logging areas (223) and in red maple (Acer rubrum) among fallen logs near margins of woods and clearings (83). The first nest in central Ontario found by Harris (225) was in a fen with cotton grass (Eriophorum vaginatum), black spruce (Picea mariana), leatherleaf (Chamaedaphne calyculata), bog laurel (Kalmia polifolia), and sphagnum moss. In Manitoba, nests have been found on bogs with sundew (Drosera) and pitcher plant (Sarracenia) (7); in Alberta, in forbs or short grasses, or attached to rosebuds (Rosa) approximately 15 cm above ground (85); and under ferns (Pteridium) among dense grasses in Michigan (25). Nests are more common in aspen woodland than muskeg in Alberta and British Columbia (7). Nests have been found in small openings among spruce, sundew, pitcher plant, orchids (Arethusa), buckbean (Menyanthes), calla (Calla), coralroot (Corralorhiza), moccasin flower (Cypripedium), sphagnum moss, Labrador tea (Ledum), bog rosemary (Andromeda polifolia), bog laurel, and leatherleaf (7). They have been found sometimes within 30 m of swamps in Minnesota (7). Parmelee and Oehlenschlager (13) described a nest on dry ground in an abandoned old field among tall grasses on the edge of spruce forest near Itasca State Park in northwestern Minnesota.


Construction Process

Information needed.

Structure and Composition

Cup nest, composed of fine, dry grasses, dry leaves, stalks of weeds, sedge stems, rootlets and other plant fibers, and horsehairs (83, 7, 203, 223). The first documented nest in Ontario was a cup nest near the base of a black spruce, made up of sedges (225). In Quebec, similar cup nests lacking domes were found in depressions on the ground and always near openings in the canopy; nesting materials included roots, herbaceous plant twigs, and pine needles (27).


One nest from Minnesota (interior dimensions) measured 3.8 cm deep, 5.1 cm wide; outer dimensions 12.0 × 14.2 cm across and 5.5 cm deep; nest wall 1.3 cm thick (7). In Ontario, a nest found by Harris (225) had an inner diameter of 7 cm. Seven nests found in Quebec had the following average dimensions (27): dry weight 5.3 g (range 2.0–8.3); inner diameter 5.5 cm (range 4.7–6.2), inner depth 2.8 cm (range 2.0–4.4); outer diameter 8.0 cm (range 7.1–9.5), outer depth 4.9 cm (range 3.4–7.1).

The dry mass of a nest in Michigan was 19.8 g (25).


Information needed.

Maintenance or Reuse of Nests

Not known to occur.

Nonbreeding Nests

Not known to occur.



Short, ovate.


Mean length (mm) × breadth (mm): 18.9 (range 17.6–20.2) × 14.5 (range 13.8–15.2, n = 27; Western Foundation Vertebrate Zoology [WFVZ]); 19.5 (range 17.3–21.3) × 14.3 (range 13.2–15.6, n = 39; 7); 19.0 (range 17.4–21.0) × 14.2 (range 13.4–15.2, n = 20) from Quebec (27).


Average weight of eggs from Quebec: 1.7 g (range 1.2–2.3, n = 16); average weight of empty egg shells abandoned by female 0.098 g ± 0.0003 SD (27).


Creamy white ground color is speckled, spotted, and blotched with auburn, bay, and chestnut, with underlying spots of brownish drab, light vinaceous drab, and light Quaker drab. More boldly marked than the egg of the Kentucky Warbler (Geothlypis formosa). Sometimes heavily blotched or clouded with wood brown (7) speckling and markings on either end, but concentrated near the larger, round end (83, 224); almost identical to the eggs of Mourning Warbler (Geothlypis philadelphia) (83). In Quebec, eggs were predominantly a creamy color with "old rose" spots concentrated towards the end of the egg (27).

Surface Texture

Smooth (7).

Eggshell Thickness

Mean empty eggshell weight: 0.11 g (range 0.09–0.13, n = 27; WFVZ); 0.09 g ± 0.01 SD, n = 5 (27).

Clutch Size

Clutch size is 3–5 eggs (10); mean 4.0 eggs (n = 6; WFVZ, JB, DJ). Clutch size in eastern Quebec averaged 4.4 eggs (range 4–5, n = 7) (27). First nest described in British Columbia had 5 eggs (reported by Phinney in 94).

Egg Laying

Information needed.


Onset of Broodiness and Incubation in Relation to Laying

Beginning of incubation is unknown, but was assumed to start before the clutch is complete in Quebec (27).

Incubation Patch

Female has a single incubation patch (109).

Incubation Period

Eleven days in Quebec (27).

Parental Behavior

Incubation is by the female only (83, 27). The female approaches the nest by landing 10–13 m away and quietly walking the remaining distance through the underbrush (203). During the night, females spent an average of 9 h 4 min incubating eggs in Quebec, arriving at the nest around 20:38 and departing at 05:42 the next day (n = 7 nests, 27). Total daily incubation lasted an average of 9 h 43 min per day from 06:00–20:00, with an average bout of 51 min 54 s. An average of 3 h 13 min was spent away from the nest per day; these times were consistent throughout the period of incubation. Females tended to spend more time at the nest in the morning and late evening, and this trend was consistent throughout incubation (27).

Hardiness of Eggs against Temperature Stress; Effect of Egg Neglect

Information needed.


Information needed.

Young Birds

Condition at Hatching

Information needed. Altricial and nidicolous, on basis of descriptions of birds that were 2 to 3 days in age (13).

Growth and Development

Average measurements of nestlings at 6 and 7 days of age (n = 3): tarsus 18.4 mm (range 15.0–20.0); wing 27.8 mm (range 18.0–35.0); culmen 6.3 mm (range 5.1–7.4); tail 3.6 mm (range 2.0–7.0); mass 10.9 g (range 9.8–12.4) (27). Nestlings achieved thermoregulation in 7 days (27). Information needed on development and growth rate of nestlings.

Sex Ratios and Sex Allocation

Information needed.

Parental Care


Brooding was originally thought to be by the female only (83). We now know that males and females participate in parental care of nestlings, but in different capacities. Only females brood young at night. In one study, females (n = 16) averaged 7 hr 48 min on the nest, arriving around 21:01 h and departing the next morning around 4:49 h (27). Although both sexes participated in brooding, females spent more time brooding and less time away from the nest than males; males spent less time brooding and more time feeding (see Feeding). Brooding behavior by both sexes declined with increasing age of the nestlings; males stopped brooding around 5 days after hatching and females stopped after 7 days (27). When startled, the female walks stealthily away from nest (7).


Both parents feed young at nest and after fledging (25). It is difficult to observe feeding because parents approach the nest with food, but walk the final 10–15 m to the nest (203, 129). After fledging, both adults continue to feed fledglings, with reported food items including moths, green insect larvae, caterpillars, and raspberries (7, 25, 13). However, there are differences between the sexes with regard to feeding. Soon after hatching, females continue to brood while males spend more time feeding nestlings. Later, females spend less time brooding and join the males in feeding young. Male visits with food are frequent and do not change as the nestlings age (27).

Families remain together at least 2 weeks after fledging (7). Young stay close to the ground, but come out into the open to be fed by parents, then disappear again into dense thickets. Feeding rates, amount of food brought to the young, and apportionment among nestlings are unknown.

Nest Sanitation

Although both parents usually carry fecal sacs away from the nest (25), nest sanitation is carried out primarily by males (27). Both parents were observed eating fecal sacs. Fecal sac ingestion by parents decreased over time, especially after nestlings reached 4 days of age; more efficient digestion by older nestlings may have lessened the nutritional value of the sacs to the parents (27). Females were observed eating empty eggshells (27).

Cooperative Breeding

Not known to occur.

Brood Parasitism by Other Species

Identity of the Parasitic Species

Has been parasitized by the Brown-headed Cowbird (Molothrus ater) (226). Anderson found a nest with a Brown-headed Cowbird egg near Moberly Lake, British Columbia (reported in 94).

Frequency of Occurrence

Information needed. The geographic extent of brood parasitism is unknown.

Seasonal or Geographic Variation Timing of Laying in Relation to Host's Laying

Information needed.

Response to Parasitic Female, Eggs, or Nestlings

Information needed.

Effects of Parasitism on Host Success of Parasite with this Host

Information needed.

Fledgling Stage

Departure from Nest

Two of 7 nests in Quebec produced 8 fledglings (from 9 eggs) that left the nest when they were 8 days old (27). In Michigan, nestlings depart the nest during the day (25).


Mass of 2 nestlings about to leave nest in Michigan was 10 g each (25). Nestlings that are 9–10 days old have paler, light pinkish legs; yellow tomia; inside of mouth is pinkish orange (25).

Association with Parents or Other Young

Both adults remain with the young soon after fledgling (223). No specific data on period of association with parents, but families remain together at least 2 weeks after fledging (7).

Ability to Get Around, Feed, and Care for Self

Fledglings stay hidden and dependent on parents during the first week after fledging; they become somewhat more independent during the second week out of the nest (7).

Immature Stage

Information needed. A small flock of 20–25 birds in late August on breeding grounds in northern Wisconsin (7) presumably included juveniles and adults that bred locally.

Skull ossification complete in first-fall individuals by 1 November (10). Similarly, McKinney (227) cautioned bird-banders against aging all Connecticut Warblers with fully ossified skulls as adult (Definitive Basic) individuals after mid-October, because many first-fall birds will have completed skull pneumatization by this date.

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