Connecticut Warbler Oporornis agilis Scientific name definitions

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



Breeding Range

The Connecticut Warbler is an uncommon and localized breeder from northeastern British Columbia (38) and southwestern Northwest Territories (39), east through north-central and central Alberta (40), central and southern Saskatchewan (Saskatchewan Breeding Bird Atlas), central and southern Manitoba (41), north-central Ontario south to the northern shore of Lake Superior and Sault St. Marie (42), and west-central Quebec east to Lac St. Jean and Chicoutimi (43). Breeding range includes the northern Great Lakes region of the United States in northeastern Minnesota (44), northern Wisconsin (45), and northern Michigan (Upper Peninsula and northern Lower Peninsula) (46).

Current descriptions of the breeding range may underestimate the true distribution across North America. The Boreal Avian Modelling Project (47) used Maxent software to model species distributions based on relative habitat suitability, climate variables, and land cover in the boreal forest of Canada and the northern United States. Their models found more suitable breeding habitat and occurrence data for the Connecticut Warbler than previously described limits of the breeding range. The actual breeding range may extend farther north in both eastern and western Canada. For example, Hennigar et al. (48) reported an extralimital record near Happy Valley-Goose Bay, Labrador based on call notes recorded during the breeding period (16 May–10 July). Wells et al. (49) estimated that over 80% of the breeding range is located in the boreal forest of Canada, similar to earlier estimates of 90% by Sodhi and Paszkowski (50) and 92% by Cheskey et al. (51). Winger and Pegan (52) estimated that the migration distance between the centroids of the breeding and overwintering ranges was 6,111 km.

Overwintering Range

The Connecticut Warbler overwinters in South America, although the exact limits of the overwintering range are poorly defined. Earlier authors (e.g., 53, 54, 55) typically defined the overwintering range to include both northern South America and Amazonia, but these and works often included records of transient individuals (or potential transients) recorded during October, November, April, and May (outside mid-winter). In an analysis based largely on sightings, Paynter (56) commented that based on limited records, the Connecticut Warbler seems to overwinter mainly on the edges of the Amazon basin. Subsequently, Hilty (57) considered the Connecticut Warbler as a transient in Venezuela, with specimen records from 23 September‒31 October and late April‒8 May, and Ridgely and Tudor (58) reported the species as mainly or entirely a transient north of southern Amazonia. In southeastern Peru, reported to be a rare passage migrant (59); e.g., four individuals mist-netted at Tambopata Reserve from 3–17 November were believed to be transients (29, 60).

There are very few mid-winter records in Brazil, where Sick (61) reported the species on the Rio São Lourenço (Mato Grosso) in southwestern Brazil in December and January (e.g., AMNH specimen 128089). More recently, additional records suggest that the Connecticut Warbler is a regular overwintering resident in Bolivia. At least 16 individuals were documented in late December 2012 and late January 2019 in the upper Machariapo Valley (1,200‒1,500 m) of northwestern Bolivia, suggesting that other inter-Andean valleys in the region could harbor overwintering populations (62). Herzog et al. (63) reported two birds observed in mid-February at Tucavaca in the Chaco of southeastern Bolivia, and two individuals mist-netted in early April at Santa Cruz de la Sierra in central Bolivia; they summarized additional unpublished reports from nearby locations in the Bolivia Chaco (Estancia El Cañon, Fortín Ravelo, and Palmar de las Islas). A study that placed geolocators on Connecticut Warblers from across the breeding range found that recaptured individuals (n = 9) were estimated to have overwintered in an area that encompassed the northern Gran Chaco ecoregion, including southwestern Brazil, eastern Bolivia, and northern Paraguay (64), though, at present, confirmed records for Paraguay are needed. The species could overwinter elsewhere in South America, but most well-documented records (e.g., photo, specimen) fall within migratory periods. Two interesting exceptions include a 9 December record at Laguna Pedropalo in central Colombia (65) and a 19 January record from Puerto Rico (see eBird).

Historical Changes to the Distribution

Because the Connecticut Warbler is secretive and breeds and overwinters in remote areas of boreal and tropical forest, respectively, it remains difficult to asses changes in its distribution. Indeed, breeding and overwintering ranges have yet to be fully described. Recent additions to the breeding range (e.g., eastern Quebec; 66, 43) were likely due to an increase in survey effort lack of earlier survey effort and probably are not evidence of range expansion. In Minnesota, other than the loss of the small nesting population in Isanti County, the distribution changed remarkably little over the past 100 years (67, 44).

Toews (68) studied habitat suitability in the western boreal forest for several species of warblers with breeding ranges largely or exclusively east of the Rocky Mountains. His modeling found a lower amount of suitable (but presently unoccupied) breeding habitat in the western boreal forest for the Connecticut Warbler as compared to Magnolia Warbler (Setophaga magnolia), Palm Warbler (Setophaga palmarum), and Wilson's Warbler (Cardellina pusilla). As a result, these eastern species have greater opportunity for future range expansion in Alaska and the Yukon compared to the Connecticut Warbler.

Influence of Glacial Cycles on Current Distribution

Zink and Gardner (69) showed that glacial cycles could have affected the historical overwintering and breeding ranges of North American migrants. During the last glacial maximum (LGM) about 21,000 years ago, their model suggests a contraction of the current overwintering range of the Connecticut Warbler. During the LGM, the overwintering range was restricted to northern South America and west to include parts of Ecuador and Peru. It also extended northward into parts of Central America as far north as the west coast of Mexico and Guatemala. There were not enough data to model changes in the breeding range. Until the entire overwintering range is described, it will be impossible to interpret new overwinter records as evidence of changes in distribution.

Climate Change and Future Distribution

Ornithologists have also begun to model long-term changes to the ranges of North American breeding birds in response to climate change. Alterations and weakening of current barriers to range expansion by climate change may provide opportunities for colonization and range expansion. One barrier is the northwestern cordillera in western Canada that contains the uppermost regions of the Rocky Mountains, Mackenzie Mountains, and Selwyn Mountains. Based on a comprehensive analysis of current climate, paleoclimate, migratory strategy, and life-history characteristics, Stralberg et al. (70) predicted that this barrier will be reduced by climate change. Boreal species such as the Connecticut Warbler would then extend their breeding ranges into this boreal region of Alaska. The expected probabilities of occurrence for this species in the Alaska boreal region were 14.6% from 2011–2040, 77.3% from 2041–2070 and 96.3% from 2071–2100.

Winker and Gibson (71) also predicted that future populations of the Connecticut Warbler at the northern region of its breeding range may expand north and west into western Canada in response to climate change and global warming. They hypothesized that species such as the Connecticut Warbler, with breeding ranges that project north and west in Canada, are likely candidates to continue in those directions as the climate warms, making suitable habitat more available. They base their prediction, in part, on examples such as the current breeding range of the Swainson's Thrush (Catharus ustulatus), which now occupies areas of the Northwest Territories and British Columbia. These areas may have been unsuitable during glacial times.

Although range expansion to the north and west may occur in response to climate change, other parts of the breeding range are expected to decrease in size or disappear. Stralberg and Bayne (72) constructed bioclimatic niche models that predict changes over the next century to the breeding distribution of the Connecticut Warbler in Alberta. Their model shows that future refugia for this species in Alberta would be reduced to 47% of its current core area from 2011–2040, 10% of the core area from 2041–2070 and 0% of the core area from 2071–2100. The extent to which similar losses would occur in other parts of the breeding range needs further study.

Distribution of the Connecticut Warbler - Range Map
  • Year-round
  • Migration
  • Breeding
  • Non-Breeding
Distribution of the Connecticut Warbler
Connecticut Warbler, Abundance map
The Cornell Lab logo
Data provided by eBird

Connecticut Warbler

Oporornis agilis


Relative abundance is depicted for each season along a color gradient from a light color indicating lower relative abundance to a dark color indicating a higher relative abundance. Relative abundance is the estimated average count of individuals detected by an eBirder during a 1 hour, 1 kilometer traveling checklist at the optimal time of day for each species.   Learn more about this data

Relative abundance
Breeding season
Jun 21 - Jun 28
Non-breeding season
Not shown
Pre-breeding migratory season
Apr 26 - Jun 14
Post-breeding migratory season
Aug 16 - Nov 1
Note: Seasonal ranges overlap and are stacked in the order above; view full range in season maps.
Seasons timeline
Learn more about seasons

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