Connecticut Warbler Oporornis agilis Scientific name definitions

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

Movements and Migration


A long-distance Nearctic‒Neotropical migrant that breeds in North America and overwinters in South America. Its secretive behavior and preference for dense undergrowth have made it difficult to define migratory routes, migration timing, and stopover ecology and behavior. Information needed on dispersal and site fidelity.

Dispersal and Site Fidelity

Natal Philopatry and Dispersal

Information needed.

Adult Fidelity to Breeding Site and Dispersal

Leston et al. (126) conducted a 23-year study of occupancy rates of several boreal species in northern Alberta. They recorded whether a species returned each year to the same point-count station to assess the impact of forest harvesting, forest regeneration, and El Niño events. Connecticut Warbler was among seven species affected by El Niño. Following strong El Niño years, Connecticut Warbler, Bay-breasted Warbler (Setophaga castanea), Common Yellowthroat (Geothlypis trichas), Tennessee Warbler (Leiothlypis peregrina), LeConte's Sparrow (Ammospiza leconteii), Lincoln's Sparrow (Melospiza lincolnii), and Alder Flycatcher (Empidonax alnorum) were more likely to return to the same breeding station they had visited in prior years. Extinction probabilities, defined as the failure to return to a point-count location that had been visited in previous years, were less likely for these species in years with a high El Niño index. More information needed.

Fidelity to Overwintering Home Range

Information needed.

Migration Overview

This species migrates later in both spring and fall than most other New World warblers. Generally rarely detected by birders in migration owing to its elusive behavior and tendency to occur in dese understory habitats, but bird-banding efforts usually reveal larger numbers of migrants (e.g., at Cape May, New Jersey; 127).

Chapman (109) believed that in spring the Connecticut Warbler migrates northwest through the Mississippi Valley, and in fall it moves east across Canada, then south along the Atlantic Coast. Research by McKinnon et al (128) and Hallworth et al. (64) using geolocators and eBird data supported Chapman's idea with some modifications. Fall migrants initially leave the breeding grounds and migrate to the Atlantic Coast, followed by transoceanic flights, first to the Greater Antilles and then to overwintering grounds in South America (see Migration Routes, below).

Timing and Routes of Migration


After departing the breeding grounds, migrates southeast towards the Atlantic Coast. Fall migration was originally thought to involve movement along the Atlantic coast with birds later moving on to the Greater Antilles and eventually to South America (109). However, McKinnon et al. (128) finally provided concrete evidence of transoceanic flights during fall migration using geolocators. They found that the Connecticut Warbler makes an approximately two-day long, overwater flight from eastern North America to the Greater Antilles, followed by a second flight of 600–800 km across the Caribbean Sea to overwintering grounds in South America. Arrival on Caribbean islands occurred from early to mid-October (128). This timing roughly corresponds with that of records on Bermuda (mainly late September to mid-October; eBird), and the 75 individuals that were grounded there during Hurricane Emily on 26 September 1987 (120). Arrivals in South America occurred towards the end of October. Hallworth et al. (64) also used geolocators to follow Connecticut Warblers (n = 9) from four natural populations across Canada and in Minnesota. Based on their data, the average arrival on the Atlantic coast was 10 September (± 6.63 days). The average departure date for the long-distance flight over the Atlantic Ocean was 10 October (± 5.82 days), with an average transoceanic flight lasting 3 ± 0.65 days. The average arrival date on the overwintering area was 9 November ± 3.52 days. Both McKinnon et al. (128) and Hallworth et al. (64) speculated that some birds do not stop in the eastern Caribbean and fly directly to South America.

Peak southbound migration in the western part of the breeding range begins late August; latest departure dates range from early to late September in Alberta (85). In British Columbia, adults may leave breeding areas as early as mid-July followed by first-fall birds in August (93). It departs breeding grounds in Minnesota and Michigan late August–early October (67, 129); migrants have been captured from 20 August to 20 September in southeastern Minnesota (130). At Eau Clair in west-central Wisconsin, individuals were killed by tower collisions between 23 August and 12 October (up to 300 in one fall season and 140 in a single night), though some individuals are recorded as early as mid-August and as late as late October (102). In Ohio, it arrives by the last week of August and peaks 10 September–5 October; the latest records in mid-October (131). Rare from early September to late October in Kentucky (132). In Massachusetts, late August–early October, with most records along the coast, but some from Berkshires, Connecticut River valley, and west of Boston (112); average arrival of 19 September on Cape Cod (133). It occurs more rarely in northern New England and the Maritime Provinces during fall. There are several records from late September to mid-October on Brier Island off the west coast of Nova Scotia (134). It occurs along the coast of New York largely from mid-September to mid-October (113). In coastal New Jersey, peak numbers pass from mid-September to mid-October (135, 127). Occurs mainly from second week of September to first week of October in Pennsylvania (136). Birds were banded in eastern Maryland from 25 August–21 October (137). Eight first-year and four adult migrants were captured from 14 September–23 October at the eastern shore of the Virginia National Wildlife Refuge, with the estimated mean migration date of 29 September (138). Farther south, fall records become more scarce in Georgia and Florida, mainly along the Atlantic coast (139, eBird). Fall migrants are rarely recorded in the Mississippi Valley (south of Minnesota) and Gulf coastal states (eBird). For example, it is a rare fall migrant in September in eastern Nebraska (140), and very rare in Missouri from mid-August through early October (141). It is a rare fall migrant along Gulf coasts of Alabama (142) and casual in eastern Texas (143). The latest fall record in North America was a first-fall individual recorded on 11 November 2019 in Scarborough, Maine (144).

Rare, but regularly recorded in the Caribbean in fall migration, September to early November (145), with records in the Bahamas, Cayman Islands, Cuba, Hispaniola, Puerto Rico, St. Barthélemy, Guadeloupe, and Barbados (145; see eBird). A first-fall female was banded at Harrison's Point on Barbados in November, where it is considered a transient species (146). It is considered an occasional fall migrant during October on Guadeloupe (147) and nearby La Desirade (148). Although records suggest that Connecticut Warbler is a rare fall migrant in the Caribbean, geolocator data (128, 64) indicate that stopover may occur more regularly there. Indeed, it seems likely that the species is under-detected owing to its elusive behavior and association with dense understory.

It arrives in Aruba, Curaçao, Bonaire, and Venezuela from late September and October (57, 149). Several early to mid-October records are from Aruba; observed in Curaçao from 9 October–13 November; and on Bonaire from 2–15 October (149). It passes through the northwestern coast of Venezuela until early October (121); four individuals were mist-netted at Tambopata Reserve in southeastern Peru, but only from 3–17 November, suggesting transient individuals (29, 60). An adult female, considered a possible migrant, was mist-netted near Loreto, Peru on 18 November 2012 (125). The first record west of the Andes and for Ecuador was a first-fall bird captured on 21 November 1996 (150). Paynter (56) mentioned a migration record for French Guiana. An adult female mist-netted on 2 November 2014 on an island in Balbina Reservoir in central Amazonian Brazil was believed to be a migrant because of its heavy fat and date of capture (151).

Recorded widely (but rarely) west of the main migration (see eBird). For example, there are 6 fall records in Montana, all late August (152). It is a vagrant along the Pacific Coast. A fall migrant was reported in late October on the west coast of Vancouver Island, British Columbia (153). It is a regular vagrant along the California coast from September through October (154); e.g., in the late 1900s, there were over 70 fall records in California, including the Farallon Islands, Humboldt Bay, Monterey Bay, Channel Islands, Point Reyes, and one from Death Valley (155, 156, 157). Compared to species of similar-sized North American breeding populations, western vagrants of the Connecticut Warbler may be observed less often than expected according to models from Ralph and Wolfe (158). They believed that problems detecting birds in dense vegetation and confusion with similar plumages of Mourning Warbler (Geothlypis philadelphia) and MacGillivray's Warbler (Geothlypis tolmiei) may account for the under-representation.

There are very few to no substantiated records (specimens, photographs) in Mexico and northern Central America (e.g., 159). Reported as a fall vagrant on Baja California, Mexico (160), and one report on 1 November from Clipperton Island off western Mexico (161). There is well-documented sight record for 6 October in north-central Nicaragua (162; eBird). It is a rare fall transient in Panama, late September–late October; 3 specimen records from Bocas del Toro (163, 164). There are several reports from Costa Rica (124, eBird), but these were viewed to lack convincing documentation and were considered hypothetical (165).


The timing of northbound departure from overwintering grounds is not clear, but spring transients (specimens) have been recorded in Venezuela from late April to 9 May (57). It is generally reported to migrate through the Caribbean to Florida in spring, though it is only very rarely recorded in the islands (April and May; 166, 145); it is possible that spring migrants typically fly over the Caribbean, though those that stopover would be very difficult to detect. Spring records include: an adult male banded at Santiago de Cuba, southern Cuba on 22 May (23); one mist-netted in southwestern Puerto Rico (date unknown, 167); one sighting in Dominican Republic (168); one report in Jamaica (eBird); several reports from the Bahamas (eBird); no spring records for Aruba, Bonaire, and Curaçao (169). There is one spring report from Bermuda (120).

Spring records are scattered through much of Florida, including the Florida Keys, Atlantic and Gulf coasts, but the species is rare in the panhandle (170, 139, eBird); most records are from the Atlantic Coast (e.g., a single tower kill of 25 individuals on 7 May near Cape Canaveral; 171). North of Florida, spring records suggest that migrants move northwest and north into the Mississippi and Ohio valleys north to the Great Lakes and central Canada. Migrants have been reported from early May to mid-May at various locations in northern Georgia (e.g., Clarke, Cobb, Clayton, and Murray counties) (172). It is a rare transient in Missouri, mainly from mid-May to late May, but occasionally in late April or the first week of June (141); it primarily is detected in northeastern Missouri, from the St. Louis area 11–24 May (173). It is a rare spring transient in Kentucky; most migrants pass in mid-May, rarely before 1 May (108, 132). In Ohio, it is rare to uncommon with most records from along Lake Erie; generally it arrives around 15 May, but most are recorded 18–30 May; most depart by 1–5 June; stragglers remain until the second week of June (131). In Illinois, the average arrival date for Chicago is 21 May (n = 15 yr of observation), and for Urbana, 17 May (n = 16 yr of observation) (174). Migrants were detected in southeastern Minnesota (Washington County) from 18–28 May (175, 130).

Most individuals arrive on breeding grounds mid to late May; in Minnesota, most arrive in late May (67); in northern Michigan, mid-May to early June (129); in Wisconsin, after 20 May, but one early migrant was collected in Milwaukee on 25 April (102). Males arrived on the breeding grounds in south-central Quebec from 27–29 May followed by females from 3–7 June (27). Peak arrivals occurred by the second week of May in Ontario (176) and mid to late May in Alberta (85). Migrants were believed to reach Saskatchewan by traveling west from Manitoba (177). A similar pattern occurs in northeastern British Columbia where migrants enter the province by moving westward from northern Alberta (91).

The species is more rarely reported along the Gulf and Atlantic coastal plains during spring (eBird). It is also more rare in spring in western and central Pennsylvania, mainly in third and fourth weeks of May (114, 136), and during late May–early June in western and central New York (113). It becomes even more rare farther east (see eBird); e.g., it is accidental in late May near Boston, Massachusetts (112) and a vagrant in spring in New Jersey (178).

Spring vagrants may appear to the west of the main migration range (see eBird). However, there are exceedingly few well-substantiated records in spring migration from Panama to Mexico. In Belize, there is a sighting from 9 May 2013 at Mountain Pine Ridge (179); it is considered a rare transient, most likely to occur on the cays in spring migration (180). A sight report from Half Moon Cay, Belize on 7 May 1958 (181) was later considered insufficiently described (182). Sight reports from Costa Rica (124) were viewed to lack convincing documentation and were considered hypothetical (165). Two sightings from islands off the north coast of Honduras on 17 March and 27 March in 1959 were mentioned by Monroe (183), but are questionable given the lack of supporting details and the early dates.

There are 6 spring records in the eastern half of Texas from 30 April to 19 May (184, 143). It is very rare in Louisiana, Arkansas, Oklahoma, and eastern Kansas from May to early June (37, eBird). It is very rare in spring in eastern Colorado (185, 186, eBird), and rare in eastern Nebraska (140, 187, 188). Five spring records (late May and early June) were reported by Marks et al. (152) for Montana; one was reported from Oregon in late May (189). It is a vagrant in California from late May to late June; records since 1958 show occurrences on Farallon Island (California), the southern Pacific Coast, and Death Valley (190, 156).

Other Migration Records

A juvenile (Formative) male was mist-netted and banded at Aldeia da Cuada, Fajãzinha on Flores Island in the Azores, Portugal in October 2019 (191; eBird). This record is the first one from the western Palearctic. It was one of several North American vagrants (e.g., Red-eyed Vireo (Vireo olivaceus), Swainson's Thrush (Catharus ustulatus), Common Yellowthroat (Geothlypis trichas), and American Redstart (Setophaga ruticilla)) found in the Azores following a series of low-latitude depressions and hurricane Lorenzo that year (191).

Migratory Behavior

Fidelity to Migratory Routes and Stopover Sites

Information needed.

Movement Behavior During Stopover

Information needed.

Duration of Stopover and Migratory Periods

Hallworth et al. (64) used geolocators to follow warblers (n = 9) from four natural populations across Canada and in Minnesota and estimated stopover duration at various locations during fall migration. The average stopover on the Atlantic coast prior to transoceanic flight was 10.5 d ± 2.31 SE. The average stopover in the Caribbean or in South America was 10.7 d ± 2.43 SE (64). McKinnon et al. (128) reported an estimate of 5‒7 days stopover in Cuba and Hispaniola.

Hallworth et al. (64) found that migrants took an average of 81.5 d ± 5.23 SE to complete fall migration from breeding to overwintering grounds.

Control and Physiology of Migration

The Connecticut Warbler is known to be hyperphagic during migration, may fatten excessively (109), similar to other migrants where fats are made up of 16-carbon and 18-carbon fatty acids (192). There was fat deposition on the furculum and belly of a spring migrant in southern Cuba (23). An adult female migrant captured in Brazil weighed 13 g, its furcular hollow filled with 30% fat (151). A majority of males and females arrived on the breeding grounds in Quebec with little or no fat (27). No significant differences in body mass or mass gain was found between between adult (Definitive Basic) and first-fall (First Basic) fall migrants at Long Point, Ontario (193). The fall vagrant from the Azores, Portugal had a body condition similar to fall migrants at Manomet Bird Observatory in eastern Massachusetts. The fat score of 3 and body mass of 14.1 g of the Azores warbler was almost identical to the mean fat score of 3 (n = 100) and mass of 14.2 g (n = 100) of Connecticut Warblers during fall migration at the Manomet Bird Observatory (191). Fruit made up 10% of a fecal sample obtained from one fall migrant on Block Island, Rhode Island (194).

La Sorte and Frank (195) were interested in whether climate change might affect the flight of fall migrants. They used 3 October–8 November as a time frame for the transatlantic fall migration of the Connecticut Warbler based primarily on eBird data. During this period, they found birds are currently likely to encounter winds from different directions and wind speeds at different latitudes and altitudes that could affect the transatlantic flights of these birds. Stronger westerly winds were found at more northern latitudes while easterly winds were stronger at lower latitudes. La Sorte and Fink (195) simulated how these winds might change as global temperatures increase. Their simulation projected a decrease in the strong westerly crosswinds at higher altitudes and latitudes where these birds begin their fall migration. They speculated that Connecticut Warbler and other species with transatlantic fall migrations could benefit from milder westerlies by expending less energy during flight and experiencing less mortality during fall migration.

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